If I were to make a prediction, I'll still pretty confidently say the Switch will sell more than 100 Million, the concept was already proven to be a hit and I'm sure many people will be willing to upgrade for the chance to play the newest Nintendo games in a new light and to also replace older Switches. I feel like the transition will be fairly similiar to the GB to GBA where it's just an upgraded device that everyone loves and would want especially when the new games come in. GBA though likely would've sold around or over OG GB numbers if its life wasn't cut short by the DS.
Alot of people may point out the other times Nintendo did simple upgrades & how the sales fell dramatically (Like from the NES to SNES or DS to 3DS or from Wii to Wii U). However, I don't think those situations will happen with the Switch 2.
The SNES sales dropped because of heavy competition from Sega, this time around I find it highly unlikely Switch 2 will receive any legit threats just like the Switch 1. Anyone who believes these portable PC consoles are threats are deluding themselves, and I have doubts despite the rumors that any handheld PS or Xbox releases will be any threat to the Switch mainly because I don't think Sony or Microsoft will invest into those handhelds as independent gaming platforms and would rather market it as just port machines from their older consoles, there won't be any exclusivity to those handhelds for people to flock over to them and take away Switch's marketshare. Even the PSP selling 80 Million did not steal significant marketshare from the DS which sold 154M.
For DS to 3DS sales drop, it happened mainly because a large portion of gamers went to smartphones for those games and the 3DS didn't stand out enough like the Switch did to offset that drop. That won't happen with the Switch 2 either where people see it as standing out enough compared to dedicated handhelds with its hybrid ability.
However, I do have doubts on whether the Switch 2 could sell as much as Switch 1. One of the reasons is that it likely won't have the Covid advantage like the Switch 1 had. I know people don't like to see people disregard Switch sales as being a fluke cause of covid, but the lockdowns definetely helped Switch at least somewhat get a boost in sales it wouldn't have if it weren't for the lockdowns. Commonly I see many people skip out on buying consoles because they don't see it as worth it to spend hundreds of dollars on a system they won't have much time to play, the lockdowns gave millions of people more time than ever stuck indoors with nothing to do, it made sense why many people who probably wouldn't have ever brought a Switch buy one since now they have that time. God forbid unless another lockdown like that happens, it's less likely where going to see those same people who won't have time buy the Switch 2.
I could also see some people being content with their original Switch as their main hybrid option and won't feel the need to upgrade just cause it has 12GB of RAM. When the Switch first released in 2017, it was a completely new and surprising concept that might've convinced some people to buy it by how amazing it looks. Now in 2025 the Switch is a standard and the Switch 2 doesn't really offer anything that's as surprising so people won't feel as much of a need to upgrade outside of that.
Also even though I said it's unlikely for their to be a true competitor to Switch 2, the reality is is that we never know. I'm sure when the NES was at its peak many people expected Nintendo to be untouchable until Sega came in and gave the SNES some true competition. Same could be said for the GBA where its life got cut short cause of Nintendo fearing the PSP. There still could be that possibility that maybe Sony or Microsoft actually do release competitive handhelds in the long term future and gives the Switch 2 some trouble, I doubt it but you can't disregard that possibility either.
However, in terms of things that could help the Switch 2 surpass the Switch 1 in sales, I believe that Nintendo making efforts to expand into regions beyond their dominant ones like in the "Other" regions starting during the Switch era has came a long way and if that trend continues, we might see the Switch 2's potential market being even bigger than Switch 1, increasing the possibility it outsells it. Stuff like the Mario & Zelda movies/series releasing can go a long way in increasing Nintendo's appeal on a broader scale.
I also think that the hype and potentially stronger "word of mouth" going into the launch of the Switch 2 compared to the Switch 1 can overall increase the hype surrounding Switch 2's brand to start and help it reach more people. We have to remember before the Switch 1 Nintendo had practically zero momentum going into it with the failure of the Wii U, that won't be the case with Switch 2.
I also do believe 3rd party support will be stronger on Switch 2 then Switch 1, and it'll finally help Nintendo potentially gain more of an image of being a console you can go to for 3rd party games as well. I think games like COD and Halo could give Nintendo big strides towards that direction and potentially help market the console to many more people.
To summarize though, I expect it to initially sell faster than Switch 1 at launch since Nintendo is going into this gen with more momentum to start but do worse in the later years compared to Switch 1. Still selling over 100 Million but falling just short of Switch 1.