This is a debate that doesn't have a solid answer if only because it's hard to show evidence of a thing that never happened. Thankfully, we don't have the metrics to compare lockdown vs. no lockdown effectively. But overkill? I can't get on board with that. Things are not good now, but I shudder to think how much worse they could be.
The current death demographic only is what it is because of the shelter-in-place, so the models we have of what it might have been otherwise are speculative. The economy was never going to be stronger as a result of this, so that's a strawman argument — either businesses were getting shut down or they were losing members of the workforce to death. There's not really an option where people continued to work, didn't get sick and fostered a thriving economy. Mental health... well, two schools of thought here. Sure, heightened anxiety and depression and substance abuse rates are coming out of this. But would the same stresses not have been amplified if people were feeling forced to be out in the workforce every day, scared they might contract the virus and spread it to their families, their friends?
The issue with these debates is that they're so often framed in the context of a world without isolation AND without the virus, which is just not realistic. Hard to prove anything definitely one way or the other, but I have much more faith in our collective ability to rebuild a functional economy and dig ourselves out of a tough spot than I do in our ability to withstand a highly contagious and fatal disease by pretending like it didn't exist. Better to do too much than not enough, always.