But this isn't like a new thing. Nintendo always has multiple updates to its portable systems. Updated systems have never been a problem for them for selling the next gen. And I think Switch 2 launch is much more likely in 2024, so it'll be more like 3 years which is a lot of time. Also Switch 2 is going to be wayyyy more capable than an upgraded Switch. Upgraded Switch is gonna run the same games but with apparently 4k using DLSS, while Switch 2 is going to be a far more capable system in general. Your worry would be like saying people won't upgrade to PS5 and XSeries because they just came out with XBX and PS4 Pro a few years ago.
1) I do think there will be plenty that bought PS4 pro and Xbox One X that won't upgrade for a while because of those recent purchases. Most don't want to spend hundreds of dollars on consoles every 2-3 years.
2) Lets review those past upgrades. GameBoy Advanced SP had a nicer screen and more convenient form factor, but I don't recall it upgrading resolution/graphics. DS Lite didn't have more power either. I don't think DSI did either, just was quicker if I recall? New 3ds had a little more power, but only a couple games came for it exclusively.
Most people who buy a system don't buy it in the first couple years anyway. By the end of this year there will be about 75 million people who already own a Switch. Only a few million of them will upgrade to the new Switch model. An upgraded Switch won't affect Switch 2 sales several years later at all, it'll just increase Switch sales.
Also I think you are way overestimating what the upgraded Switch is. It's not running 4k native. It's not gonna be some vastly more powerful model. It is supposed to use DLSS to have 4k resolutions, which means it only has to be a bit stronger and I guess have more RAM. Sounds like this upgrade will be very on par with New 3DS. And do you really expect it to have games exclusive for it? I wouldn't expect any games to be exclusive for it. Anytime Nintendo has made an upgraded system (GBC, N64 expansion pack, DSi, New 3DS) there have been like 2-4 games that you needed the upgrade to run, so at most I would expect 2-4 exclusive games, but I think zero exclusive games is much more likely. The only possibility I see of any exclusive games would be because Nintendo said it will feature some new forms of interactivity, so there might be a few games that use that and can't be played on OG or Lite. I wouldn't expect any games to be exclusive to the upgraded Switch just because of increased specs - increased specs is just gonna make games go from current resolutions to 4k, or possibly clear up any framerate issues in whichever games have them. I think you are confusing the idea of 4k with thinking that means it'll play PS4/XB1 level graphics - that is not what anyone is talking about and there should be no expectation of that, that is what should be expected of Switch 2, not a slightly upgraded Switch model.
Don't let the 4k confuse you. DLSS allows that without a huge spec increase. This should on par with any of Nintendo's previous handheld upgraded models. There is nothing to suggest thing will in any way hurt next gen sales like 3 years afterward. Just like nobody expects PS5 and XSeries sales to be hurt from Pro and XBX.