The big problem I see is that BTC is the reference for all other coins... If an altcoin peaks in a bubble, and starts to drop, no problem, sell it for BTC, and switch to another coin or hold the BTC, and your profits are safe. But if BTC drops, the only option to keep the profits is to convert to Fiat.
For those holding altcoins, all of which are referenced to BTC, their prices are nosediving as everyone switches to BTC, and many have lost a little bit (some a lot) of value in Fiat over the last few months. If BTC crashes to say 25% of its current price, it doesn't just affect those holding it - it affects anyone holding any coin at all - and I'm not so sure that the altcoins will regain their value in Fiat if that happens...
In other words, the way I see it at the moment, BTC goes up, and the value of the crypto market as a whole creeps up, but only due to the rise in BTC as the alts fall every time BTC goes on a run (I assume from people switching to BTC). If BTC goes dramatically down (loses 50% for example), the entire crypto market drops 50%... I just can't see BTC holders jumping into altcoins to avoid a loss when the value of those altcoins is measured in the very currency they're trying to get out of.