since battlefield would become horiffically unbalanced, should buildings be leveled with ground, fully destructible environment isnt an option, but this does not mean buildings should be immortal to damage.
how the bombed battlefield would look at the end, when no more damage can be applied, can be seen in birch grove (burning moscow village), houses are ruined, but they still do provide a fair cover for infantry.
i remember playing berlin test, there was an objective made out pretty much of nohting but sandbags, and when those were destroyed, attackers has to crawl the ground while still not having enough protection (despite of this, it wasnt that hard to attack most of the time). Devs made this an exuse for not making buildings ruinable, however, this is not a case, should some wrecks be still left after the bombing…
at the same time, i have to say, buildings should not become pile of rubble upon receiving 50 kg bomb or something alike, it should take a lot of doing to bring them down (depending on size ofc, buildings may even consist of multiple smaller stages which are ruined one by one…), but it’d really be a good addition to the game
remember of times before we got ability to dig and bombs didnt make holes in ground? game felt so static… ruinable buildings will not only bring more tactical possibilities, but also new ambience to the game instead of “ah, there is a thin sheet of wood above me, im safe”
looking forward to hear your opinions 
and as always, i could’'ve forgotten to say something so feel free to ask, shall something not feel right.