This question is a little more tricky, since Sony has been a lot more secretive about their upcoming console and have really given us no concrete details about backwards compatibility.
Mark Cerny, chief architect of the PS5, initially mistakenly said that only most of the 100 best selling PS4 titles will be made backwards compatible. Following public outcry, Sony later corrected that to "most PS4 games".
While the oddly specific first criterium would still leave us quite confident about GTA 5 falling into the pool of selected games, this is hardly a definitive yes, especially with the corrective statement in tow. If GTA 5 does become backwards compatible on the PS5, it might benefit from PSVR.
Microsoft isn't thinking about VR when it comes to the Series X for now, so this would be Sony's riposte to the whole "automatic-improvement" pledge their competitors have made. For now, fans who want to experience the game in VR can only rely on PC mods.
We imagine that the result of GTA 5 not being available on PS5 would be pretty ugly for the console. This would also depend on how the game's sales figures look like when the new consoles launch, but considering that over the past few years GTA 5 has only sold more, instead of declining, we don't think it will drop off that quick.
A game of this magnitude isn't one you just brush off without feeling some hurt.
Since we are speaking entirely in hypotheticals, it is hard to answer which will be the ideal platform for GTA 5. On the one hand, we don't even know if it will be available on PS5, and if yes, what kind of enhancements — if any — backwards compatible games will get on the platform.
Even if we operate on the assumption that yes, the PS4 version of GTA 5 will be playable on PS5, we're still looking at the Series X as the ideal choice. Guaranteed compatibility with a "remaster" out of the box is hard to beat. We might be swayed if GTA 5 was PSVR compatible on PS5, but there's no indication to believe that yet.