I think it will sell less for sure, but nowhere near a disaster.
The market circumstances are completely different, with the market overall becoming increasingly splintered and segmented, and there's no COVID-pandemic to drive unnatural second-wind turns in anyone's lifecycle this time around (even previous gen consoles had a lift). Add to that some confusion around naming convention and design for the more casual (yeah, I know, some will kill me for using an illegal word) consumer, and reaching the Switch's numbers seems lofty.
Static entertainment products and electronics are having as hard a time as ever, just about the only way of keeping units flying is refreshing and going for revisions (this is the same for all consoles). We'll also see how pricing affects it, large parts of the world are in an insecure phase economically right now (again; this applies to all other machines as well). Yes, phones are still selling well, but it's an entirely different world; the smartphone still has by far the highest perceived value of any consumer electronics product. But even smartphone sales are down in the past 3-4 years, overall.
The market for consoles overall is significantly smaller than it was in the peak 7th generation. The total sales of the Switch+PS5+Xbox SeriesX/S is about 260 million, the 7th gen home consoles sold nearly 240 million, with another 220-ish million units from the handheld segment. That's about 460 million in total, while the 9th will be lucky to cross 300-310 million at this rate, the market for dedicated consoles has shrunk dramatically, in spite of the industry overall having grown immensely over the same period of time. I believe the industry is about five times as big today, in revenue, as it was when the Xbox 360 released, that puts the sales numbers of 7th gen of consoles into even more perspective. To further drive home the above-mentioned economical point, the industry only grew about 2.1% in 2024, according to reports, this is peanuts compared to many previous years. Many major publishers and developers are bleeding, stocks tanking, and console sales were rather slow in general for 2024.
If I had to pick a number, I think the Switch 2, barring any major reveals or developments, will sell about 110-125 million units. But it's just pure speculation at this point. I do think it will have a really solid launch, Japan is a given, but also EU will contribute greatly.