Killer instinct 1 plays even on Gameboy. Everything is possible.
Pc is still 10times stronger as ps4pro, plays games on a much better framerate and higher res, but those games can still run on consoles. Its just by downgrading the game if they really want to do that.
It's possible in theory but practice is another matter and sometimes the downgrade process just isn't worth the effort either because it would require drastic engine overhauls (or even a new engine entirely), or major concessions to the game design itself.
It's an oversimplification, but it can be looked at allocation of processing power; aesthetic versus design. Sometimes games are conceptually very simple and not technically demanding on hardware in design, but are aesthetically incredible. DOOM is a great example, I think; there's nothing much going on in the game design that shouldn't be applicable to a wide assortment of hardware configurations, so all you're left with is scaling the asset quality which allows for a lot of wiggle room in performance benchmarks.
But take another game where hardware power is allocated to a facet of design beyond the presentation and transferring it to weaker hardware become far more challenging. Dead Rising is a good example. Even though the Wii port was junk as is, that was very much a game tailored to the CPU and memory standards of hardware the Wii didn't have a hope in hell of achieving. Dead Rising simply would not work as-is on the Wii, no matter how scaled back the asset quality might be.
FFXV could be similar, but I suspect it isn't. It's a very pretty game so I can't imagine it'd look as good on the Switch, but the game systems and design don't seem overly demanding or complicated in so much that they couldn't be scaled down to weaker hardware.
But that also doesn't account for the engine. Some engines are just built with strict optimisation for modern standards of CPU threads/cores, RAM speeds and allocation, etc, and thus scale terribly when older, dated configuration are introduced. And fair enough.
It does kinda sound like the Switch, unlike the Wii and Wii U, is a bit more tailored towards modern standards of architecture though, raw performance aside, and that should make it far more accommodating towards potential ports.