Bron was already shooting above his career 3P% by Year 2, so I think the "trashness" of his early jumper vs. his later jumper has been overhyped. It improved, but not beyond the pale. If anything, his ability to finish inside of ten feet has been what's really improved in an elite way over the years.

The other guys, more appropriate to compare to Simmons height-wise, have been extending since the moment they arrived in the NBA.

AD and Cuz both took 65% of their shots inside 10 feet their rookie years. (Bron took a mere 48%.)
Simmons is taking 73%.

AD and Cuz also both took 22.5% of their shots outside of 16 feet their rookie years. (Bron took 35%.)
Simmons is taking 9%.

Limited sample size for Simmons, obviously, but the same was true in college for Davis and Cuz vs. Simmons. They've shown more desire to take those shots. (I'd also place Cousins alongside someone like Leonard in terms of being anomalously terrific in terms of jump shot growth over the years.)

I don't doubt that he'll attempt to work on the distance jumper, but until teams can repeatedly prove they can limit his effectiveness and keep him outside of ten feet on offense, I don't see why he should start chucking mid-rangers and 3s now. You weren't drafted #1 overall for your shot-making, you were drafted #1 because you're a beast at finishing inside, and you have elite vision, handles, and slipperiness for a guy your size. Pick your spots, play smart winning basketball, and work on that free throw stroke in practice, as that should also help with the mid-ranger as a side effect.

I also don't think many defenders are going to cheat down against Simmons effectively, because Simmons has the speed/handles/vision to simply drive into or back down into the cheating defender and run the offense from the interior, kicking out to shooters or looking for cutters. Simmons is big and agile enough that he'll likely draw the help, so he'll find the open man-- and if the defense doesn't help, then you're leaving a defender one-on-one against a guy who is both really quick for his size and really efficient inside of ten feet. They'll try to do that, but the number of defenders who can slow Simmons 1v1 is certainly low. Again, if I'm his coaching staff, I'm telling him to keep doing what he's been doing.