Any game can be downported if you really want to do it. Same with R&C But it will look like dog shit.
Right, they
could do a version of Rift Apart if they really wanted to do some ugly replacements.
Like the Rift tripping sequence where he's failing between levels, that
could be simulated with maybe a video loop and some limited gameplay elements like rails and enemy characters. (So far, Rifts have only been shown as short-time connections linked with mostly forced-scrolling elements like the Grindboots rails or the creatures Ratchet rides, then he gets dumped into a locked-in battle encounter on the ship, and only certain elements of all of that need to be interactive to preserve a version of the illusion/gameplay, assuming rifts never get deeper than that.) And the yellow in-level Rifts are reusing the nearby level elements, so
could be doable too if you limit where Rifts come/go and maybe have some tunnel-loading to make sure you have the exact two points needed in memory by the time the character comes across those triggers. (Titanfall 2 had these "rift" jumps.) And the amazing level of detail and the crazy number of particles and bolts and layers of graphic effects to make things look CGI, that all
could be hacked down if not scaled in assets included in the port. Anything
could be demade, if you want it bad enough and are willing to suffer the difference envy...
It just gets a little silly in this case to try. At some point, though, until the PS4's software market dries up, Sony will be taking a look at the consumer demand for titles and picking projects to somehow get out to that base. I don't think we'll get many exclusive "little sibling" games like we did on past-gen versions (partly those were fueled by the existence of PSP and sometimes Vita, so I don't see how we'd get the equivalent of like Motorstorm Artic Storm or Jak&Daxter Lost Frontier while PS5 dives into the deep end.) But a lot of PS5 midrange titles, or maybe some of the big titles that could go back into redevelopment post-release (maybe with a porting studio) for a lesser version. Heck, we're already getting games like Spider-Man MM and somehow Horizon FW ported over (the latter of which I'm a little concerned about; Spidey was always sort of a juiced-up PS4 engine/assets project, but Horizon is a full sequel with an improved Decima engine and so we'll have to see how Guerrila handles the versions and if things like rumored flying mounts can be done in either/both projects.) Don't be surprised if it happens; just demand that your next-gen version is the best it can be.
(*And all of the porting effort wouldn't "hold back" the next-gen version. It always depends on your engine/tech methods and your multiplatform approach and your manpower/management/budget structure, but ultimately any downport is the downport team's problem, and it just depends on the priority of that product on the market, or the needs of cross-version online compatibility, as to whether the downporters make the high-spec version contend with those problems.)