While I believe that the way the system works, if a unit "takes a hit", this means that it will at pinned at the very least. If it doesn't become pinned, then it might get killed outright, and if that doesn't happen then it might retreat instead.
I think - but someone can correct me here if I'm getting it wrong - but if a unit gets "pinned" in one round, then it tends to remain "pinned" in subsequent rounds (which makes it less likely to kill something, but possibly also less likely to die due to the soldiers "hiding"). I think "pinned" may possibly be handled a bit differently for vehicles, but I'm not sure.
At any rate, I don't think it's common for a "pinned" unit to take another "pinning" hit - or this may even be outright impossible? - but I believe it can go from "pinned" in one round to "retreating" (routed) or dead in a subsequent round.
And yes there are feats that can convert a "kill" into a retreat, among other things.
So TL;DR: I don't disagree with you, but I'm skeptical as to how useful this kind of counter will end up being. If I'm interested in "resilience" I'd also want to know in what round it retreated/died, how many attacks it was under each round (and by what enemy), and if any "special" thing changed one result to another (and when) - and all of this information would be difficult to summarise on the results screen. This information is however available on the detailed results view, though I rarely look through it as it is VERY detailed - though I do wish it were more obvious exactly how/what scores constitute a kill, retreat or pinned (it's easy to see the unit get hit, but how the "severity" of the hit is determined does seem a bit obscure to me).