There are already a few buildings that get completely destroyed, including ones that collapse.
I think you may be missing the point regarding the building being destroyed idea. The point isn't the fact of the building getting destroyed, the point is the extra gameplay element it introduce, and how can it provide new elements of excitement and perspective. I'll elaborate to help.
Currently, there are small buildings (more like Huts) that gets destroy in BF3 and there are some multi-floor buildings that have parts of it crumbling (like the ones in Metro), but there aren't any multi-floor buildings that completely topples over or gets completely destroyed to the ground.
Imagine this scenario - You and your team are in the middle of town under team death match rules. Most of your opponents have taken over a large 7 floors building. Some of your team members tried to rush into the building but only a few got so far before being shot to death. Taking aim you tried shooting your opponents from the outside, but your opponent has the upper hand because (1) they are on higher grounds, and (2) the window frames and walls provide good shielding for them. To top it off, outside doesn't have much "cover" areas making you a sitting duck.
So what do you do? You and your many team members decide to grab some tanks and attempt to obliterate the foundations of the building. THe building shakes and booms, which are communication to let the player know that the building's foundations are being hit. After a while, the building starts toppling over risking everyone - including your team mates who may be inside - to be crushed once the building falls.
When the building begins to topple, the players inside the building will notice this as their viewing perspective will be altered (naturally) along with a big booming cinematic noise of something falling over (think Lords of the Rings, the section where Frodo and Aragon had to jump over a pit of lava). From that point, ALL players have approximately several seconds to either (A) jump out the windows or off the roof of the building and parachute to safety, (B) run out of the building via the exit, or (C) make their own exit using bazookas and SMAW type weapons.
Once the building hits the ground, ALL players that were inside the building will die as they will be crushed or slammed against a wall to death, and players that are in the falling path of the building from the outside will be crushed.
Now the building is toppled to the ground, players can still battle the fallen structure like BF3 Aftermath. Your opponents no longer have the higher ground advantage, but there is now a new playing area to battle on which provide new areas of advantages and disadvantages.
Hopefully you now understand. To be honest, I thought things like this would be in Aftermath and more... I wanted more innovation. Like above, the same same could apply to the earthquakes... I thought massive earthquakes would occur dynamically at ramdom and in-frequent points in the game and change your fighting surroundings, thus providing new areas to battle in. These ideas, plus the others I mentioned earlier, are innovative or at least compelling and I thought DICE would always try to add something innovative to every pack. Even a new campaign would be nice! However, all we got in every pack so far were new maps, weapons and vehicles. Quick frankly, I think that is getting stale.