For any altcoin to do the job as well as Bitcoin does it, that altcoin must have too much mining power for an attacker to outvote the network.

Are there "many alternative cryptocurrencies" with hashrates high enough that I couldn't get a couple hundred thousand dollars worth of hardware and double-spend with impunity?

Which blockchain are miners throwing computation power at? That's the one that you can actually have confidence is atomic and irreversible.

Right now, that's Bitcoin.