Exactly this ^ Because this pretty much defeats the whole purpose of the shared estate feature because your basically a resident of the house.
I was sadly involved in this problem with a friend and watching her house (and all the time/work/gil we put into it) just vanish right in front me =/ (though some friends of a friend of mine were able to move in, it still sucks.) One way they could help with this is to give shared housing the ability to reset the timer but by accepting the shared housing feature you forfeit your right to buy a house because technically your already apart of one (gotta have give/take somewhere)
Leaving the shared house will give you your right back and put the house back into a state of demo after time has exceeded the limit