Absent some fundamental flaw, it's going to be very hard for any alt cryptocurrency to catch up with Bitcoin given the head start it has.
A fundamental flaw in bitcoin will extremely likely affect the altcoins as well. Most of them are forked from the same code base, and are only different in a few points.
Also true in a non-technical sense: if it turns out that bitcoin has a big problem and thus suffers a blow big enough to derail it, people will be talking of cryptocurrencies (at least the block-chain based ones) as a failure for a long time to come. Maybe the idea will be picked up again in 50-100 year, but none of the current altcoins will 'take over' bitcoins place.
