Not a biologist, but been around them long enough. I been cultivating a theory that mammals strategy is more or less "go fast; break things". That is, asteroid or not, we were on track to out evolve everything that came before by extending and exploiting explosive cell growth. No animal converts matter & energy to body-mass faster than young mammals, the cold blooded do not stand a chance. The competition would get larger eventually only if some adolescent mammal did not eat them first as peers. consider how you would bet on a 1 year old raccoon vs a 1 year old alligator then again with a 5 year old raccoon vs a 5 year old alligator

The mammal strategy traded the long game for the quick win.

We have all sorts of (genetic) machinery who's function is to grow fast but then STOP as we can't support gigantic sizes. (Maybe in part because of the quick and dirty foundations but thermodynamics have scaling issues too)

So we need to shut of all those machines before we get too big.

And so here we happily sit in our bodies, our mammal factory and everything is great; just don't push that big red button over there, or pull that lever or that one or that one or ...

eventually if you live long enough some of those buttons will be pushed and the mammal machinery will do what it evolved to do. grow fast and break things.

A final observation on humans in particular, recently bipedal, have not gotten all the bugs worked out on that. Deployed a monkey patch for dimorphic gender singling requires cron job flipping on a subset of growth machinery some years after the main global shutdown but only flip them on for a season or so; still seeing evidence of switch bounce issues, see ticket BCRA1.