If bitcoin exists only to cure monetary inflation and any guy can spin up an attempt at inflating it, it's not very good at it's only purpose, is it?

Of course, those premises are incorrect. Bitcoin exists to create a peer to peer electronic cash system. Curing monetary inflation is only a part of that. And these other things exist primarily to provide alternative peer to peer electronic cash systems based on peoples idea of what is an improvement. They're not fungible with bitcoin and therefore cannot inflate bitcoin, the only way they devalue bitcoin is by attracting users that like their feature set. The free market at work, for money, competition based on merit, it is natural selection. If bitcoin is the best and the only thing worth using, they'll have little impact I'm the short term and virtually none long term.