Cody Bellinger is not a very good comp for Kepler. Bellinger's a first baseman who hits like one; if you put him in CF as a tactical move, say to get Max Muncy's bat into the game, the hope is that Bellinger's bat really outshines other CFers and compensates for a marginal glove. (I still don't understand LA's roster well enough to see why you wouldn't put him in a corner OF spot and use someone else in CF, but it may just go to show that all of this is second-order effects.)
Kepler may yet achieve that level of hitting, but right now he's pretty much the opposite player. If he's not using his OF skills, a lot of his value is wasted.
I am all for positional flexibility. But from a given player's perspective, it's to give him somewhere to play when the team has 3 better outfielders, say, than himself. We aren't close to that in the case of Kepler, or at least I don't like Cave or Rosario better than him.
Flexibility is good in the abstract, and if (say) Cron goes down with an injury mid-game, and Austin is DHing already, telling Kep to go grab a first-baseman's glove for the remainder of the game sits just fine with me.
But while an overall flexible strategy is fine, it still comes down to specific games. And if I'm writing out a lineup that has Kep at 1B, an instant improvement is to swap him to an outfield spot and let the other outfielder be positionally-flexible and handle 1B duties that game. That will remain true, even if Kep improves at bat and starts to hit like a first-baseman too, until the day his OF skill eventually erodes.