However, now there are many alternative cryptocurrencies that accomplish the same thing.
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 outspend.
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.
Yes, however, if you are only using it for a transfer of value and not a store of value, any crypto currency will do as long as it has not been hacked, and so far, none of them have been, without the community knowing about it. Correct?