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.
I don't think you understand how the Switch Pro would be able to accomplish 4K resolution. This isn't a "pure" 4K like the PS5 or Series X does. It would use DLSS, which is essentially a high-tech upres solution, and would only effect the Switch in docked mode. It would take the current Switch games in 1080p, and display it in up to 4K through DLSS.
In handheld, it will still only be up to 720p, so it won't be much more capable than the current Switch and Switch Lite. This is why I don't think there will be any games that will be compatible only with the Switch Pro.