The TOS crew had gotten old, but they had also written that into their stories. From TWOK's "Young. I feel young!" line all the way to TUC where the characters retire! They knew they were getting old, and they worked with it.
TNG ignored it and had Picard playing Superhero all the time.
I agree. As long as they had created coherent action filled happy stories, they would have succeeded way more than they did, no matter who the cast was.