I think we already know Danny's answer to this - he's gone with the young guns.
But I think the OP asks the wrong question. See, most win-now teams don't have the opportunity to develop younger guys. They have late first round picks about every other year and a smattering of 2nd rounders. They're forced to take on minimum vets because they lack any other options to do so.
Danny's playing a different game. He's playing a win now and win later game. Plus, he knows he has enough vets to cover the minutes. In fact, he has a minutes crunch on this team. He doesn't need other vets. Look at the roster breakdown from last year:
IT - 34 minutes per game
Hayward - 34
Crowder - 32
Horford - 32
Morris - 32
Smart - 30
Brown -17
Rozier - 17
Baynes- 15
I'll save you the math. That's 243 minutes. Now add in 20 minutes for Tatum, because the #3 pick is going to get on the floor. That's 263 minutes. There's 240 minutes in a game (48*5 = 240).
Of course, this doesn't account for injuries. So run the same analysis for total minutes played last year (which would factor in any injuries last year), giving Tatum 20 minutes per game for 70 games (1400 minutes). You wind up with 19,744 minutes played. There's 19,680 available via math (48*5*82) and the entire Celtics team played 19,755 minutes (which includes rounding) last year.
Guys like Brown and Rozier are expected to have more minutes this year. Brown is likely to come close to 2,000 minutes this year (+500 conservatively) and Rozier should top 1,500 (+250). So now we're at 20,500 minutes played when there's only 19,680 available. And this doesn't take into account guys who are expected to contribute like Semi, Zizic and Theis.
In short, there's a MAJOR minutes crunch if you use last year's minutes played. Veteran backups are good only in case injuries are far worse than they were last year and even then there's still a minutes overload.
So why have veteran backups who wouldn't play unless the injury situation was so bad that the season was lost anyway? Instead, fill the back half of the roster with talented young guys and let's see what they can do. Let them earn minutes the hard way by going against a talented core every single day in practice. We'll see the cream rise to the top and we'll be stronger for it in the long run.