I don't know what it is that we're lacking, but I don't think it's best termed "toughness" or "mental toughness" or some such thing. Maybe just "clutchness" is best.
There's no way players like O'Neale, Gobert, and Ingles would have made it in the league it they didn't have toughness or mental fortitude out the wazoo. But they don't seem to be able to raise their level in clutch situations. DM can, and Bogey can at times, but otherwise it's not clear that we have anyone else on the team that really can do this.
I don't know if it's that they've already more or less maximized their talent such that there's simply not another level to go to, or if it's just that clutchness is just something that some players have and that others don't, or maybe it can be developed over time (ie Giannis wasn't clutch, until he was)? As
@fallenchicken implies, probably the only hope I have in switching to Ainge is that he might be better at finding clutch players. Too bad that it will probably require a tear-down to get there.
At any rate, we have one more playoff season to hope the team somehow finds it, though I'm certainly not expecting that they will. I'll be rooting for them, though; and I don't think this talk about "calling out" coaches or players is the path to any kind of solution until then.