Here is a completely failed 1.5" thick concrete subfloor on top of 1/2" plywood and 2x10 joists 16" on-center, throughout the entire upstairs of a condo with 3 beds and 2 baths.  The cracks generally run parallel to the joists, clearly demonstrating that the thin plywood underneath lacked the stiffness to support the load.  Interestingly, all of this was original to the home, which is an early 70's build.  (Sidebar: this same builder decided to create a long cantilevered overhang with laughable/dangerously short back span that has also failed and violates all current codes, but I might create a separate thread for that.)

With a little 20lb demolition hammer and a pointy bit, took only 45 mins per room to bust it all up.  I'm still determining the easiest way to haul it out...there's not a good way to make a chute to the dumpster.  I might just make a hole in the subfloor & ceiling below and just push it down to the first floor, then wheelbarrow it out.  Since the whole downstairs is gutted, it's no biggy if we make a mess down there.

Once it's all removed, we'll add 2 layers of 3/4" tongue and groove plywood, glued & screwed.