Personality. No matter what fans think of Raffy's defense, he's Carita, their Big Puppy, almost always in a good mood. He may not be a leader because he's not a redass like Pedroia or call profane team meetings like Papi or Sale. But nobody ever said Devers is not a good guy.
Vlad -- and this is just from what I've seen in games vs. Boston and New York on TV -- is always the first one off the bench whenever there's any confrontations between opponents, like brushback pitches or cuss words. He wants to fight, even if he's not the target. The only time we see Raffy get mad is when he makes mistakes.
And Guerrero doesn't look like he cares at first base, where he's a negative with the glove. Merloni and Youk both called him out for being unprofessional and not even trying on some plays in a game last year.
I'll take him as DH, but not to replace Devers, and definitely not to solve the problem at 1B.