Islam would do the same thing that any other religion does when threatened, reinterpretation. Somehow, Muslim scholars worldwide would convene and decide that either the Kaaba could be rebuilt elsewhere, or isn't all that important, or that it was a sign from Allah to do something or other. Just about all religions have done this. Judaism has survived for millenia without any access to its primary holy site. Christianity has survived despite being deprived of access to the Holy Land for centuries. Rome, the seat of Catholicism, has been sacked countless times, and Constantinople has done worse. Yet, all these religions survive just fine. Buddhism no longer even exists in any appreciable degree in its country of origin (India), and thrives outside of it.