Mickey isn't an upgrade over Bass. He hasn't even played a game in the NBA yet plus where would he get minutes at because last time I checked he was a 4 and Sully/ KO are still on the team.
Rozier is the 4th guard in the rotation and I don't see him playing at all with Bradley and IT on the roster Marcus can move to the 2 but Bradley and IT can't they are too small and will get taken advantage of. Rozier is 6'2 and is a worse player than Bradley and IT so he will get no minutes. Unless we trade one of those 2 guys. I don't even see him averaging more than 5 min per game this season
Bass is a free agent - no indication on whether he'd want to return or whether Danny feels it's worth bringing him back (since we're going young).
Sully has one year left on his contract and will probably want a max (or at least double figure) salary after this year - he's not worth that, and not likely to be retained. Good chance he's traded by the deadline.
If Bass and Sully go, suddenly Olynyk becomes the only PF on our roster - and a guy like Mickey could full the bakcup role nicely.
Bradley is the most valuable player on the team in terms of trade value (aside from maybe Smart), so if Danny has any dreams of making a major trade, chances are Bradley will be gone.
If Bradley goes, then a guy like Rozier seems to have the skills to be able to do anything Bradley can.
If Bradley goes we may also need backups at both PG and SG spot, as Thomas and Smart likely become our starting guards. Rozier could be out backup SG, and Thornton could be a backup PG.
There is a lot of flexibility - many backup options in case any of the above scenarios occur, and any of those guys walk (or get traded).
Danny has a chance to play all 4 rookies during summer league and preseason, see how they go...keep the ones who he feels could produce at an NBA level, cut/trade the others.
If some of these guys pan out better than expected, it gives him more flexibility to consider trading some of our existing veteran players.
For example, if we find that Rozier can do anything Bradley can, then how much cap space can we potenially save by trading Bradley and replacing him with Rozier?
Likewise Bass (who we'd likely need at least a MLE to bring back) if we can replace him with Mickey?
Flexibility, flexibility, flexibility. That's what this is all about.