Nintendo/Nvidia can't make a portable device advanced enough to last through PS5/XSX generation; the gap is too big. However, they can make a device that runs Switch games at higher resolutions and framerates, maybe with some other improvements like draw distance and textures, and uses an OLED panel. Such a device would make Nintendo's games look very nice, and might also be able to run a few XBO/PS4 ports that a standard Switch could not. I think that's as far as they'll take it.