A little flex never hurt anybody—unless you’re talking about my back after a long day, but that’s another story. I’m right there with you on the “poke it with a screwdriver” test. If it’s just a bit of give and no real rot, I’ll slap some screws in and call it a day. Honestly, half the decks I’ve seen have a little bounce here or there, especially in older places.
But yeah, moisture is where I draw the line too. Had one unit where the subfloor looked fine until I pulled up the vinyl—turns out it was basically compost underneath. That was a fun call to the owner... Not every board needs to be replaced just because it creaks, but if you see black spots or feel that spongy give, better safe than sorry.
I’d say trust your gut (and your screwdriver). If it feels solid after patching, you’re probably good for years. Just don’t ignore those weird smells or soft spots—those are never “character,” no matter what the tenants say.