I thought Tau just used markerlights to make that ruin irrelevant.
I like the idea, Roger! I'd probably avoid making it too terribly lethal to be caught in the collapsing building. I know, I know. Buildings falling on you are a big deal, but turning every ruin into a death trap will just punish units that rely on cover to function properly (snipers or mandrakes for instance).
I like the idea of making it be a wound on a 4+ (representing that the danger posed by the building is severe enough to threaten even big things like
MCs, but also random and chaotic enough for people to crawl out relatively unscathed. I'd say either make it
Ap - or
AP - with rending and pinning. You're already leaving a unit without the advantage of their ruins, so wiping the squad out with the resultant damage seems unreasonable.
Also, what happens when the ruins are destroyed exactly? do you replace the terrain/models with a different set of ruins? Just pretend the walls aren't there any longer and force everyone onto the ground floor? I'd lean towards the latter and simply say that the terrain now counts as rubble rather than ruins. Wait, Rubble *is* less helpful to be in than ruins, right?