Each of the last three years we've gotten 30+ games at catcher from guys who weren't one of our top two catchers heading into the season (Amaya in '23, Nido/Bethancourt in '24, McGuire last year). Amaya especially is made of glass, I don't think we should write off Mo getting some run at catcher.
He of course could just hang out at Iowa until the IL trip that opens up that playing time, but A) is it really possible for him to radically change our defensive evaluation on him between now and like July and B) to your point of growing pains we're just kicking the can on getting those out of the way.
I just don't see the point of blocking the kids for low margin upgrades. Especially at DH, where we're not boned if Mo faceplants. Austin projects really well, Long purely offensively is probably a bit better than Mo (at least in the short term), or we could of course just rotate guys through it as a rest spot. And of course I think KC's got a great point that DH is about as easy of a spot to fill at the deadline as you could ask for.