Stuff wise, it's hard to see why he couldn't start. 95.5mph touching 99.4 as an RP probably becomes 93 touching 97 as a SP, so still very good.
Command is obviously there.
The slider and the changeup both look good to me. Slider seems to get an elite whiff rate. Fastball gets an above average whiff rate in the pen.
He would profile/project well as a starter, I think.
The entire thing hinges on mechanics. If they have good reason to think he won't hold up as a starter then they should leave him be. I think he does have ugly mechanics... but I don't know anything.
There's also maybe some hangover from concerns surrounding him as a prospect. He was always referenced as someone with a "bad body", and then he ended up getting hurt. Not that he's out of shape anymore.
It's really not a cut and dry question and I don't think anybody from "outside baseball" can answer it with any competency.