You've been saying this about the Bane pick since the offseason---but repetition doesn't make it so. They could have drafted Bane and dumped Green or Carsen. That was a mistake, one several of us noted at the time. I get the theory you and others had---they wanted someone who knew the system, minutes might be an issue etc. Those were reasonable arguments at the time---but this far into the season we know they were wrong, don't we? I'm far from a reactive Ainge-killer but it's important to be realistic looking back, too.
I think this would be reasonable if FA happened before the draft, but going into the draft basically everyone was talking about how the Celtics really couldn't use 3 picks without a stash and the reason is this:
1. Everyone knew that certain roster spots were set:
Tatum, Brown, Smart, Kemba, RWIII, GW, Theis, Langford, 2 picks, and backup/starting C (ended up Thompson, but they were always either getting one in FA or keeping Kanter). That leaves 4 spots assuming they could dump Poirier. At the time they had the following: Green, Carsen, Semi under contract.
2. They needed players in those spots that they could cut outright to make a Hayward S&T deal happen
3. The final roster needed to go into the season with a 15th man (if not 14th and 15th) that could easily be cut for buyouts
The hope was likely that 2 of the last 4 spots would be filled by Hayward and a vet PG (a clear need given they had none on the roster and Kemba was going to miss time). leaving only 2 spots.
However, there was also the need to leave 3 spots open in case of a Hayward S&T, because that was always going to be at least 2 if not 3 or 4 for one.
So really there was at most 1 fungible spot (likely Carsen, which would mean eating the salary of a guy you just drafted 33rd). And that assumes they would be willing to cut Semi, which didn't seem that likely given how much they used him the previous year and liked him as a matchup dependent defender specialist.
If you put a 1st round deal in that spot, you don't leave yourself many options for buyouts, trades etc.
Now as it happened, they didn't sign Hayward, or get anything back for him, they missed on the first buyout (Batum).
Right now Green/Carsen are basically placeholders for the TPE and buyout options. PP came out of the gates way better than anyone hoped, and Kemba had no real setbacks, so Teague has become expendable, but at the time of the draft, there was a real roster crunch. That the post-draft moves turned out poorly, doesn't change the decision making at the time.
There is also next year to consider. Right now the Celtics have 9 guys who are no doubt going to be rostered next year barring trade, that's 11 with next year's 1st and at least 1 TPE, they'd likely want to bring back Theis, that's 12. It never really made sense to add the #30 pick, a player who is likely around the same level as Carsen to them, at the expense of flexibility.
This trade also amusingly ties back to where we got the pick..... #20 and Baynes for MIL pick and Carsen. A deal that made some sense at the time, but turned out poorly (losing a good player, and a better pick to free up cap space that we ended up not needing because we never went under the cap and worked a S&T instead). It's a domino effect, we made a dubious trade that ended up not working out, and in the process gave ourselves a situation where we had to make another low return trade.
So the MIL pick trade was not a bad one at the time. It is more of a necessary move that looks worse because the subsequent moves were failures, than a move that was bad in and of itself. Now it was made necessary by a combination of poor roster/pick management and bad luck, and was followed by more poor roster space usage, but the move itself was basically inevitable, I don't know that any good GM wouldn't have made it if dropped into that spot.