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.
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.
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.
You don't even understand how mining difficulty works.