At this point all the hardcore people have basically already bought a Switch (over 80 million sold). The vast majority of the remaining 30-50 million buyers are either repeat buyers (malfunctioning hardware so buying another one, made a silly decision to sell Switch and want to buy one again, want multiple Switch's for convenience/family) or casuals that take a long time to come around. Casuals don't care about Pro models, so they want wait to buy a Switch before now and when the Pro comes out, and when the Pro comes out they will probably just be like "cool, I guess I'll get that one" and not think about it too much.
If Nintendo announced a Pro model around the time they came out with Lite then this would be a problem. But we are now 4+ years into the life of the system. This can only help them propel Switch sales.
There may be some truth here, but the 4K Switch Pro won't be like the New 3DS which is a bit faster but where most of the new software is still compatible with the original 3DS, this is going to be close to a 2 generation leap in graphics. I don't see how the original Switch is going to be getting much (if any) of the software that is being developed for the successor as you will basically need to create 2 separate games. I don't see how even semi-casual gamers will be as likely to want to buy a handheld that may not be getting new games in 2022 other than maybe a sympathy port of BotW2 like the way that the Wii U got BotW.
I am not saying I want to see the OG Switch get phased out. In fact, I think that it is quite likely that Nintendo may not reveal new hardware at all at this point like people are expecting. Nintendo may let the original Switch run its course and once sales start to dip below 300k per week, reveal the new model. Keep in mind that this may not happen this year.
The new device, in my opinion, will primarily be nice for people that have 4k TV's. I don't think there will be many games released on it that take tremendous advantage of DLSS and, therefore, cannot be built for the OG Switch/Lite. The device will only be relevant for roughly 2 years (by the time it releases) before the actual successor to the Switch comes out, and by then the vast majority of 3rd party developers will have already moved on to making exclusive content for that way more powerful device. The average time between pro devices (GBA SP, DSI, New 3ds) and their successors (DS, 3ds, Switch) are less than 2 years and we have no reason to believe Nintendo would break from their historic pattern without evidence. I just can't see them designing something that is "2 generation leap in graphics" only 4+ years into the life of an incredibly successful device. If this was a Wii U situation that would be understandable, but it isn't. They will confuse and annoy many of their consumers if they essentially abandon the OG/Lite Switch platforms in favor of developing exclusive content for Switch Pro.
If Switch Pro really is that much more powerful, I don't think Nintendo will make any exclusive games for it. And if Nintendo doesn't do it, I really can't see 3rd parties doing it much either. They know they won't have the selling power without major Nintendo exclusives launching alongside them for said devices. There will be a few exclusives to the Pro, probably no more than 1-3 Nintendo games and 10-15 major 3rd party games, but that's probably it. They know they will only have 20-30 million Pro devices max by the time this generation is all said and done that they can tap into, so why bother developing many games that small of a market can actually play?
If Switch Pro doesn't get revealed until next year (such as in the hypothetical you stated near the end of your comment), then it probably won't launch until end of 2022 or perhaps Spring of 2023. If they launch it Spring 2023 then I suspect it will simply be their next device, rather than a pro model, as that will be 6 years after Switch's Spring 2017 launch.