I think 2021 will be the last BIG year for the Switch in terms of major releases and megaton hardware sales.
Next year we have Monster Hunter Rise, Mario 3D World Deluxe, SMT V, and likely Breath of the Wild 2, among others.

Then, after 2021 all of Nintendo's major studios (Mario, Zelda, Animal Crossing, etc.) will move on towards their Switch 2 projects. From that point on the Switch will be sold primarily on a few things.

1) The back of its massive library of games, spearheaded by the evergreens. - All of the Switch's biggest selling titles, from Mario Kart down to NSMBU Deluxe, pretty much their top 10-15 best selling games + RFA just continue to sell gangbusters and drive hardware sales for the system. BOTW 2 and SM3DW will be two more feathers in that cap. And possibly even Monster Hunter Rise, which I'll get to in a bit.

2) Continued B-C Tier 1st Party Support. - Now, we'll still get 1, 2, maybe 3 major A-S tier titles for the Switch from 2022-onwards, Pokémon Gen 9 being one of them. In fact, I'm pretty sure the majority of them are going to be Pokémon. But for the most part, most, if not all, of Nintendo's major first party franchises: Mario Kart, 3D Mario, 2D Mario, Zelda, Animal Crossing, Splatoon, Smash Bros., and Pokémon. They've all left their mark and some have had numerous entries on the Switch. Now, those franchises will shift gears and jump towards the next system. So don't expect any more HUGE titles beyond what has already been announced for the Switch. But that's alright because as I mentioned previously, those evergreens will keep selling the system for as long as it's on the market, and the Switch will be on the market for a very long time. Now, as for the smaller franchises: The lesser Mario spinoff (Mario Party, Mario Sports, etc.), Kirby, Yoshi, Donkey Kong, 2D Zelda, Fire Emblem, Pokémon Mystery Dungeon, Metroid, Xenoblade - or whatever Monolith's next project is,. Games that are, by no means, major, blockbuster, megaton, system sellers like the A-S tier titans I mentioned earlier, but they're still very important to Nintendo's plans and bottom line. They're the games that keep the Switch's player base active and engaged, they fill out the year's release schedule, and only serve to add to the Switch's library in quantity and quality. Those games are still being made at a steady, consistent rate and they'll continue to be done so for the Switch throughout the rest of its cycle.

3) Rising 3rd party support. - Going back to Monster Hunter Rise, I think every single user on this site knows that game is going to be the biggest selling 3rd party title on the Switch (Take a bow, Capcom.) It is going to do monstrous numbers in Japan, obviously. And I also think its overall, worldwide totals can give MH World a run for its money. The Collector's Edition of MH Rise was charting in the top 10 on Amazon US as soon as it went up for preorder and now its listed as "Temporarily Out of Stock" and we're still 5 and a half months away from its release! I think 10 million is a lock for the game and I even think 15 million is a good possibility. And that would make it the best selling 3rd party game on a Nintendo system ever. Or 2nd if you count Tetris on the Game Boy (Nintendo owned the publishing rights at the time, so I count it as 1st party.) That would be incredible on its own. But I think this leads to a bigger deal - I think this game is where the floodgates will open for 3rd party support and 3rd party companies are going to take their development for the Switch to another level, especially Japanese 3rd parties. Monster Hunter Rise (Capcom), Dragon Quest XII (Square Enix), possibly even Persona 6 (Atlus). Nintendo can feel comfortable and relaxed with pulling back on their own major support for the Switch, because the 3rd party companies will be there to pick up the slack.

4) Hardware Revisions and Price Cuts. - Just the natural flow and occurrences of the video game market. Nintendo will obviously introduce new revisions with the Switch just like they have with damn near all their hardware, dating back to the NES. Basically, Switch Pro (I'm still calling it Switch+ because I think that's a damn good name!) Possibly a Switch TV, etc. And as time goes on and sales start to decline in conjunction with these new hardware revisions introduced, Nintendo SHOULD drop the prices in their current models - the Lite and the Hybrid, accordingly. I envision that probably by late 2021, we'll have a Switch+ released at $300, while the regular Hybrid and Lite models will be cut down to $170 and $250 respectively. So that they'll have the high-end model for those who want to upgrade and their current two models at lower prices to get the late jumpers in.

Assuming these 4 factors are perfectly executed, I see the Switch having a gradual, natural decline, with its successor releasing no earlier than March 2024. And I not only will it reach 150+ million units sold, it will become the best selling system of all time.

They would need to postpone the Switch 2 long enough to achieve that. I don't think it's going to happen, they don't want to fall too much behind techwise.

Not going to happen, simple as that
Its less than halfway to 150 million after 3.5 years

Its had the perfect year this year with both of the current consoles about to be replaced and covid, which has been a boom for all things IT.

Now both of these points helping the switch are about to go away, yet somehow switch sales will stay or even improve over the next 5 years
Plus also Nintendo will release the switch successor within a couple of years

The switch is doing better than I thought it would but 150 million is a near impossible mountain to climb

I mean...that's not accurate.

No one said Switch sales will stay or even improve from this year. This year Switch could hit 28 million (or more), which would put it at what like 77+ million. Three more years of 28 million and then zero sales after that would put it at 161 million. So no, it doesn't need to sell as much or more, it just needs to continue doing very well and Nintendo needs to give it strong support through late in its lifecycle.

If they say replaced it end of 2024 and kept supporting it with good first party software and smart decisions like price cuts into 2024, a reasonable path to 150 million would be: 29mil/78mil 2020, 24mil/102mil 2021, 18mil/120mil 2022, 14mil/134mil 2023, 9mil/143mil 2024, 4mil/147mil 2025, 2mil/149mil 2026, 1mil/150mil 2027.

I don't think it will happen, 135-140 million feels much more likely, but it is possible, and the Switch successor is a long ways away it's just about first party support late into the lifecycle, continued increase in 3rd party support, game price cuts, new models, and hardware price cuts.

I think Metroid Prime 4 is generally underestimated. Imagine if it will have a compelling online multiplayer mode. It could become BIG!

What does Nintendo need to do to reach 150M Switches sold you ask? Simple, they need to sell 1m more than 149m.
I don't think it will happen tbh, but I wish them luck.

What indication has Nintendo given that Switch will be replaced by a successor in less than two years?

Their recent statements on the matter have explained that Switch is in the middle of its lifecycle. 

Define "BIG!" 

I don't know if Metroid Prime will ever become big because of it's dark and isolated atmosphere, which turns more casual gamers off usually. And tbh I hope Retro/Nintendo don't sacrifice the atmosphere of Metroid Prime by adding a online multiplayer mode just to make it sell better since one of the best things about Metroid Prime is its atmosphere.

Well, obviously not CoD, Overwatch or Fortnite-level of BIG but it definitely could attract all kinds of online-shooter-lovers owning a Switch. If the online part is really compelling I can see that it will be frequently played online for years. This would definitely attract additional buyers. This, in combination with my hopes that Metroid Prime 4 will be Breath of the Wild-level-great, the game could possibly sell 10 mil. lifetime and sell systems.