Making a portable ps5 would be a waste of time and resources. And there would be less incentive from devs to participate in such a thing with the fact that it just more sense to do game streaming directly from the console to a portable like this. Mobile architecture is just way to different to the one we use for consoles but powerful in their own right.
Sony already has everything someone could want for a portable. They have a massive library of portable titles from the Vita, PSP, backwards compatibility with ps1 and they could add ps2 to it, mobile titles and already had the endearment of the indie community.
They let go of all of that thinking their failure with the vita was due to it being too niche and let Nintendo run to town with a console hybrid [They tried that with the vitaTV] that beats them in sales every month almost. And instead chased the VR market which is actually niche for real, has limited devs, has limited interest and requires that you be already invested in Playstation, meaning that it does not bring substantially new players into the ecosystem.
The Switch itself is the reason Sony won't make a new handheld until it can at least hang around with the PS5 and both can play the same games.
Nintendo raised the bar for the portable space, and Microsoft is the only one so far to compete and they are side-stepping it via Xcloud using a device everyone has (Smartphones).
Sony will either go the route of Microsoft and expand PS-Now to the level of XCloud, or go the Nintendo route and make a proper hybrid system.
And if they do the latter it will have to play the PS5's games otherwise they run into the same problem Nintendo did with the Wii U and 3DS with their First-Party companies having resources and projects split between the two systems.
If they can both play the same games with the only major effort to make it work on either one is optimization rather than fundamental programming, then that helps keeps Sony's current model.
Sony
will not go back to a split-dev model no matter how hard people want it because they are too used to the single-dev model right now, and even Nintendo making the Switch shows that it is a better model to do than dividing resources between Console and Portable games.
So the next Sony portable, if it ever comes, will have to cross-compatible with PS5 games without much hassle in order to keep that.
It's a sad thing for Portable PS fans, but it's the objective fact of the matter.
