Earlier quoted context omitted.
Warren Buffett is buying Alphabet out of neceessity - he already sold most of what he could sell and has way too much cash on hand - he just need to buy 'something' - Alphabet seems like the least overvalued big stock out there.
Is he required to buy stock? AFAIK he could just as well hold cash or bonds, and if he believes a bubble pop is imminent (within the next couple months), that would be a responsible move.
Its so big that it could take with it whole global market.
There is no safe asset you can turn into in such a case, digital money would literally mean nothing when whole financial system collapses.