It's also weird to assume that it'll "drag the "legitimate" crypto currencies down". It's much more likely that the legit ones will be a "safety" asset and only gain in credibility with large investors.
It's funny because bitcoin seems designed to fail eventually. The computational cost of mining is going up faster than hardware can keep up. That's a problem because maintaining the whole blockchain is dependent on the continued possibility of mining. When the mining becomes impossible it will be impossible to maintain the distributed ledger. But that won't actually happen. Instead the mining will get increasingly difficult and transaction rates will have to decline accordingly. It will eventually become unusable and of little or no value. At least that's what my (limited) understanding leads me to believe.
I used the word "legitimate" because the parent to my post did. The way I see it there is nothing legitimate about any of these so-called currencies. They are probably all Ponzi schemes in the end.
Bitcoin has a self-adjusting difficulty rate. If blocks are being produced too fast, it increases the difficulty (i.e. average computational cost to produce a block), and if they’re not being produced fast enough it decreases it. Thus, it should never become impossible to mine.
There’s also the block reward halving process, which is separate, but in theory miners will still be incentivized to mine even after it hits 0, due to transaction fees. If not, well, the difficulty will go down until they are.