Was Hitler objectively bad?

[The Omnissiunt One]

This is rooted in the same false objection answered above. Even if you don't want to call it morality, you can be objectively wrong in thinking that desire x will be the one that brings about a state of affairs in which more and stronger desires are fulfilled. It may be that you think keeping slaves will bring about a better state of affairs, in which case you are objectively wrong. You may think voting for party y will bring about a better state of affairs where it turns out to be wrong.

Using a methodology for evaluating desires we can be more accurate in determining which desires tend to lead to a better state of affairs.

I would argue that this is very much morality, but whether you want to call it that or not desires exist and are the only reasons for action that exist, and there is an objectively true or false standard by which we can judge desires by their tenancy to bring about a state of affairs in which more and stronger desires are promoted vs thwarted.

It's true that we can objectively assess what will promote the fulfilment of most desires, and that desires are the only reason for action that exist. You still have to bridge the is-ought gap by explaining why we should desire things that fufill others' desires, rather than just our own. 'Ought' only makes sense within the context of a hypothetical sentence like 'If you want to succeed, you ought to study hard'. Of course, you could say, as you have, that desirism succeeds if we wish to promote others' desires, but there is no objective reason for us to want to do so. Thus, I can't see how desirism solves any problems that are not solved by, say, preference utilitarianism, which is similar to desirism in its normative prescriptions, but doesn't claim to solve any meta-ethical difficulties.