This possibility exists even today of course, but miners seem a lot more likely to organize and
attack right after (or right before) the mainstream network kicks them in the nuts
by halving the reward.
Maybe we will see a fork of BItcoin, which keeps the block reward at 50BTC, with a very high %
I think that altcoins won't get any considerable chance to take over.
Keeping reward at 50BTC per block is bad.
Also, reward halving can work as self-fullfilling prophecy: people will expect price to rise and may stop selling BTC before the halving, causing price to rise.