There is already a lot of difficulty porting to Switch from PS4/Xbox One, so much so that most AAA games skip the platform entirely, those that don't often come late and require entire seperate teams. With the event of PS5/SX, games will also become less optimised for PS4/Xbox One
More than conceptually difficult, I'd rather say that for AAA titles it may become time consuming. Scaling down textures and, if necessary, redesign the least important game item models with less polygons doesn't affect the gameplay and game engine, but AAA titles with a reputation on graphics to defend can't just scale down with mostly automatic tools, they must check and tweak the results carefully to keep them as good looking as possible even at lower res, this surely makes managers question the opportunity and profitability of some ports, but as far as we know some ports could be simply delayed to arrive when the NS user base will be larger than now. Another factor is that a large part of NS users are mostly great fans of titles developed for portables and/or family and party titles, this can make devs and publishers predict that some PC and home console titles would attract a too small share of NS users. Obviously this evaluation may be totally wrong, and anyhow those that must decide know that even a very pessimistic prediction about the percent of NS gamers interested in a game can eventually take to an absolute number worth the investment once the total NS user base becomes large enough.
Anyhow, I see that we both agree that the more games will become optimised for PS5 and XBSX, the more difficult NS ports will become, up to eventually become impossible.
I'd add another thing: NS shows Ninty that, at least for Ninty itself, hybrid is the way, so, even with a dropping 3rd party support, unless it drops really too fast, a new NS can't come too soon, as it would make it too difficult to make it powerful enough when undocked. Luckily for Ninty, thanks to the faster growing mobile market, ARM CPUs development can react faster than Intel and AMD to new requirements.