Well, it definitely shouldn't be off a cliff steep.

Even if Nintendo is in a tough spot for next gen, they can use that gen as a filler like they did with the Wii U since they have massive cash reserves. Using the Wii U filler lead to an efficient release schedule for their games to release on the Switch, even going as far as delaying their Wii U system seller in favor of the Switch.

Which experience will Switch owners miss out on with the release of PS5 and XBoxSeX, that the Switch currently has or that the PS4 and XB1 haven't? As cool as ghost of Tsushima looks, similar experiences exists with Sekiro. Hellblade II? Well, Hellblade is even on Switch. Godfall? I have no idea what that does, but what little we saw it is an action game alike DMC or Bayonetta. VR? Currently on PS4. What else? New version of Battlefield and Call of Duty? Will they extend the formula?

So far it seems, the early lineup of the new consoles offer either only experiences the Switch has, or that current consoles have and Switch misses - and that doesn't keep Switch from selling so far. If the generation goes on, experience say the game makers start to utilize the abilities of the new gen more extensively. At that point it might offer new experiences. But that is 2022 or later. At which point a successor to the Switch will be close.

Switch will peak in 2020 or 2021, despite the new offerings from Sony and MS.

Also, it is interesting how many posters have the feeling the new gen might hamper Switch sales, but without any data or reasoning backing them up. I myself thought it might be a possibility - over a year ago. Since then sales data and game offerings have shown that this doubt was unneeded. The current wave of posts express more the hope of their writers, than a thorough analysis.

EDIT: If posters are so confident that PS5 and XBoxSeX will offer an experience that will make Switch look unattractive, it would be logical to make a bet that either PS5 or XboxSeX will outsell Switch in 2021. I am willing to bet against it, as I was willing many years back to bet against that PS3 will eventually outsell Wii lifetime. For this bet to decide we will have at least only to wait two years.

Personally, I think this year was Switch's peak.  At least this fiscal year ending March 2020.  They have a $199 SKU, now, and have released pretty much all of their big hitters.  The reason next gen may hamper it is that it will be getting fewer 3rd party titles because of the power discrepancy, and it won't be seen as the new toy on the block.  Of course, like I stated, it will mostly be because it will be getting long in the tooth and will have fewer big releases.  These things don't sell forever.  And a Nintendo peaking late in its life just goes against their own console history.  Just like people thought that the Wii was going to easily pass PS2 with how it was selling for the first 3 years, but didn't even come close, 55M+ short. 

Also, that would be an easy bet for you to win.  Of course the Switch will outsell them in 2021.  They'll be new consoles with smaller libraries.  Even the popular PS4 shipped just 15.4M in its first full year.  If the PS5 is $499, the Switch should be able beat it by 3M-5M.  If Sony works some kind of magic and can get it out for $399, Switch should still beat it by 1M or so.  2022 is when the new consoles will start to hit their stride, and Switch, naturally, will still be declining.

Why would we hope it's not a steep decline? I like my Switch. But, I would like to see Nintendo move the concept forward. If sales decline quickly, the next generation will come sooner. That's better for gamers.

(1) While it is theoretically possible that the Switch sells weaker next year, no signs are pointing towards that. Remember the PS4 in it's peak year started to have weaker holiday sales, indicating the next year sales might cool down. No such sign for Switch so far.

(2) The Switch already doesn't get all the third-party titles. Exactly that was the reason why the Switch was declared DOA in 2017 in this exact forum.

(3) Well, I constantly heard over the last two and a half years that PS4 and XB1 are more powerful than Switch, so it never was the new toy.

(4) You assume the Switch peaks naturally in 2019 so that this will be two years after peak, and you think PS5 and XBX offer something so enticing, that it drives people away from Switch. This in combination should make it easy for Sony and MS to outsell Switch in 2021.

I'd rather prefer my console generations as long as possible. I don't like the idea of changing hardware fast, if a generation can last close to a decade, I'm fine with it. That way we can actually get considerable jumps in tech instead of just small improvements.

1.  We don't have concrete numbers for other regions, yet.  However, we do know that despite it having a $199 SKU and Pokemon, it is having weaker sales in Japan this December.  Just this past week it was down ~90K YOY.  Will other regions be able to make up for this, will they be flat, or will some be slightly down, too?  We will have to see when Nintendo releases its quarterly earnings. 

2.  Switch doesn't get them all, but it has had pretty good 3rd party support.  10x better than Wii U had.  But, I have already said, this won't be the major reason it will start dropping.

3. It was still the new toy on the block, even if some (including myself here) refused to see it.  It had all the charm of the Wii, but was also portable.

4. Has nothing to do with them stealing customers, as the Switch isn't going to plummet.  It will be in a slow downward trend and PS5 and XBSX will start out slower, like all consoles do.  Especially if they end up being $499.

- Ofcourse it'll have weaker sales this December though Smash released last December.

- His point is the third party support the platform gets is what's expected for most part, AA and Indies complimenting first party titles this won't change with the new consoles as these games don't rely on the bigger power increase.

- It was released alongside new SKU of other platforms so they had new toys of their own it's also nothing like the Wii's situation tbh only similarities is that they both are appealing products which is why they have sold well the actual intricacies in why is heavily different. Wii introduced home consoles to the blue ocean DS had created while NS has brought together the two branches of the market giving buyers reason to buy it and its games even if they have other platforms.

1. You mean the week that is down YOY because last year Smash released that week? The two weeks prior it was up 66K and 79K though. Hence this graph:

Globally it is a bit muddled up, because Black Friday was at different times, 2018 on 23. November and 2019 on 27. November. So our comparison is this week in 2018 (1.37M Switch) to this week in 2019 (1.63M Switch) for black Friday. Or for the week after (Cyber Monday) this week in 2018 (884K Switch) to this one in 2019 (1.15M Switch). So both weeks are significantly stronger this year.

2. But it misses the big titles that are always conjured here in the threads as reasons for the death of Switch. It will keep getting the smaller and indie titles. A lot of Switchs 3rd-party support are late ports. That actually can stay unimpeded by the release of the new gen. I also expect another lame FIFA with missing features and so on. The loss of 3rd-parties will not be relevant before 2022, but at that time we also may look at first rumours about the next Switch.

3. Hmm, well, it will retain it's features people see value in. True though, that it might not be seen as new anymore. The problem here is, that the new gen will not replace the Switch in the regards that make it unique. So that might not be as relevant as you think.

4. Well, but we all expect it. What you say here is that the release of the new gen has no impact of the Switch life cycle, but that was the point the discussion started with. If you say the new gen has no or negligable impact on Switch sales, I fully agree.