Not saying that this is it's main problem, and other constructed languages like Lojban are probably even less relevant despite trying to be more "culturally neutral".
Even then, it's a very big jump from ‘people in some countries already know this and don’t need to learn at all’ to ‘people in some countries will have an easier time learning since they’ll recognise a lot of the words’; making this jump would already be a great improvement upon the status quo. Out of the languages which make the jump, Esperanto is by far the most widely spoken and otherwise used in practice.
But as an international language? I think having everyone learn something that is comparatively easy to learn is certainly more fair than having most people learn something that is comparatively hard to learn and understood by others from the get-go, thereby giving the latter an unfair advantage. And I feel that being more fair in this regard is indeed morally superior.
(Then again, that might just boil down to one's own idea of morality. But I feel like this one shouldn't be all too uncommon, and the claim is entirely reasonable in its terms.)
Ironically the one biggest thing which presents an ideological advantage for Esperanto is that it's native to no nation, so all the people who speak it are closer to being equals in that regard; yet it is the imperialism and a large number of native speakers which has helped to make English so prevalent. So just being technically and morally superior doesn't win you adoption by a long shot, even if you try.
And it may be of note for this post that the original author of Esperanto was very conscious of this whole formal/informal issue, which is why he opted to introduce only the plural second person pronoun into practice.