The most likely scenario is that Switch will become the best selling console of all time. This year is going to be it's peak year, and it's going to peak higher than any home system ever has. The only system with a higher peak would be the DS, and it's sales fell extremely quickly after it's peak (basically it fell harder than any successful system ever has). All Switch needs is a somewhat gradual descent in sales after it's peak and it will coast right into the #1 position. Mathematically it's very feasible. It's peaking later and higher than the PS2, so it is set up very well to outsell the PS2. Or if you want to compare it to the DS, then it just needs a gradual descent, and it will ousell the DS.
People say "the stars need to align" in order for this to happen. Well, let's actually look at what would needs to happen for the "stars to align" on the most successful system ever and see if that is happening for the Switch. Since DS is the big selling Nintendo system, let's compare Switch and DS. Here is what would need to happen for a Nintendo system to surpass the DS.
1. Incredible First Party Software. Check. Switch easily has this category down. Compare the first party line-up of the Switch and DS. Both had evergreens, but Switch evergreens are actually core Nintendo titles. Zelda and Smash were never considered evergreens before. Now they are. Mario Kart 8 has already outsold Mario Kart DS and it's still going strong. Animal Crossing is heading like it's going to sell 3-4 times what the DS version sold, which is crazy. We can safely add in 2 more Pokemon games and another mainline Zelda before the Switch is done and several more years of other first party titles. DS had amazing first party sales, but Switch is totally blowing it away.
2. Lots of 3rd Party Support. Check. DS had 1985 games. Switch already has 2665 games. Conservatively speaking, we can double that and Switch is going to have at least 5000 games by the time it is done selling. However, so far each year Switch gets more third party releases than the last. Most likely Switch will have 7000 games (or more) by the time it's all said and done. It totally blows the DS away here too.
3. Cheapest System During a Recession. Check. DS sales peaked during the 2008 calendar year which is when a major global recession hit. It was also the cheapest current system on the market. Switch will be the cheapest current system on the market when the PS5 and Series X launch, and there is a major recession looming. Since I live in the US, I've only been looking at US financial metrics, but here it appears this recession is going to make 2008 look like a cakewalk.
4. Room for Revisions and Price Cuts. Check. Switch has only had one major revision, the Switch Lite, and ironically the base model is the more popular system. Switch has also had no price cuts. We are almost 3.5 years after the launch of the Switch, and it's in short supply in many places and it still hasn't had a price cut yet. It isn't even holiday season either. That is unprecendented. People are waiting for Switch to do something unprecedented. It's doing it right now. No system has ever gone this long without a pricecut, and it isn't going to get one soon, because it's sold out in so many places. Switch has so much room to introduce revisions and price cuts and it doesn't have to do this anytime soon. Meanwhile, the DS actually was being outsold by the PSP when it first launched. It didn't start selling like crazy until after it's first revision. Since the base Switch model is in short supply 3.5 years after launch, it is blowing away the DS here too.
In short, the stars are aligning for the Switch. It needs the stars to align and they most definitely are. When you compare the Switch to other successful systems it's peaking later and higher than everything but the DS, which peaked around the same time but higher. The DS also crashed hard after it's peak and still became the 2nd best selling system of all time (not far behind the PS2). All Switch needs to do is to not crash hard. It just needs a gradual decline in sales each year after this one. How can it do that? Solid first party evergreens, lots of third party games, be the cheapest option during a recession, and have lots of room for revisions and price cuts. It has all of that. It's set up to sell better than the DS by a fair margin.
The Switch is set up to be the best selling system of all time. This is actually the likely scenario.