The transition can be changed, some people are too entrenched with traditional "rules" as if none of them can be bent or broken. It reminds of discussions of the NX/Switch on this board too where you'd explain the hybrid concept and the person would say "yeah but how's the standalone home console NX supposed to work then?".
Just because you have a Switch Pro, and even if it can play some games the previous model cannot, doesn't mean the previous model is just immediately worthless and goes into the garbage.
You could have a situation for example for example you have 200 Switch games release in 2020 ... lets say 15 of those are Switch Pro only all from third parties. Only specific titles that can't possibly be ported to the older Switch ... Kingdom Hearts 3, current Call of Duty, RE2 Remake, etc. No Nintendo games are Switch Pro only. You still have 185 games coming to the OG Switch. In 2021, maybe that number increases a bit to 20-25 3rd party games only for Switch Pro. 2022, number ramps up again and maybe Nintendo makes their first Switch Pro only title then. That's fair IMO.
And people upgrade their smartphone every year or two because they WANT to, not because they need to. The fact is today is not the 1980s anymore. People love to buy new stuff, it makes them feel good. Unboxing videos get millions of views. An iPhone 4 from six years ago can do most anything the new iPhone can, you don't *need* to have it, it's a want. And I'm pretty sure the XBox One X is the lead XBox SKU now, for PS4, the Pro is probably 30% of their sales, but that's very lucrative because they apparently make more money on the Pro model than the regular model.
Nintendo will want those revenue streams. The Switch is simply in a curious spot tech wise because if you double or triple the Switch chip like the PS4 Pro or XB1X, suddenly PS4 ports become much more doable, it's just right at the cut off point.
I also don't think going forward there will ever be just "one" Switch model once this new one arrives. There will always be two, one high end and one cheaper lower end, and it will keep cycling that way, the high end model will move down to the low end spot after 3 years or so with a new high end model taking the $300-$350 price point tier (which makes Nintendo more money). The 3DS currently is just a placeholder for the lower price Switch, once that is out, 3DS is phased out, but a new higher end Switch then takes over the top end spot.