I agree with the majority of your post, but I wanted to single this part out...
Keeping those players, how would you expect us to get the starters you speak of? Maybe Richardson, Bogdanovic, Cauley-Stein, & Labissiere become good enough one day to start, and if we keep our 1st this year perhaps we get 2 more shots at a starter with our 2017 1st & 2018 1st, but I guess my point is why would we want to hold on to guys like Collison, Casspi, & Koufos? Yes they are good back-up players, but it's not impossible to sign these players (considering we signed all 3 of them during a time when our team was dreadful). Wouldn't it make more sense to try and move them for more youth in an attempt to increase our odds at landing those starters you speak of? I think it would. Even if they don't develop into the starters we're in search of, they may still become the caliber of a Collison or Casspi or Koufos.
That's why I recommended moving Gay, Collison, & Koufos for Rondo (waive), Denzel Valentine, Bobby Portis, Jerian Grant, & Chicago's 2017 1st (lottery protected) and then turning around and flipping Casspi for Delon Wright. That leaves us with Temple being the only veteran locked up past the 2017-18 season (lots of cap flexibility moving forward). I don't think I'd trade Temple either. I'd want that type of teammate & hard worker around my young guys. Having a guy like that in our young players ears and helping them teach what it is to be a team would be very valuable. Same with Tolliver. I'd probably guarantee his contract next year so it would give our young guys another great veteran teammate to model themselves after. With that same thinking in mind, I'd probably waive Barnes even if he picks up his player option. I wouldn't want that type of personality around my young guys.
Going into next season, we'd have this team:
PG - Jerian Grant / Delon Wright
SG - Malachi Richardson / Garrett Temple
SF - Bogdan Bogdanovic / Denzel Valentine
PF - Bobby Portis / Skal Labissiere / Anthony Tolliver
C - DeMarcus Cousins / Willie Cauley-Stein / Georgios Papagiannis
***SAC 2017 1st***
***CHI 2017 1st***
Now that would be a team that could have the ability to go somewhere depending on who we take with our two 2017 1sts (and our 2018 1st as I expect that will probably be another high pick considering the roster above) and how we develop our young guys. Grant & Wright probably don't have a ceiling of a high level starting PG, but I can definitely see them as 3&D PGs in this league. Richardson can be a 3&D SG. Bogdanovic could be a sixth man type player in a Ginobli mold. Valetine could be a nice utility player in that he can do a little of everything. Sort of like Igoudala with a better shot, but worse athleticism & defense. Portis can be a decent stretch big who can score in multiple ways while holding his own on the boards. Labissiere has a very high ceiling and could end up being a go-to scorer type talent or a Ibaka type player. Cauley-Stein could end up being one of the best defensive players in the league while being a consistent lob threat & mid range jump shooter. Papagiannis could be Koufos w/ a better post game, a jump shot, & more size.
Obviously these are all "coulds," but you only need to hit on a few to have something special. Then obviously a high pick this year had the ability to yield a star player. Ball, Isaac, Fox, Tatum, &
Ntilikina could all be options and then you still have the Chicago 2017 pick and next year's 2018 pic (which will likely be another lottery pick).