My point is that the cost and revenues are not a function of each other as much as they're both a function of usage. Once you get a high volume of users, it's pretty natural to convert at least a small part of that to advertising revenue, even if doesn't fit well to your platform (cf. Twitter). It takes a principled decision not to dip into that revenue source -- I remember once reading how Jason Calacanis was pissed at Jimmy Wales for not wanting to include ads on Wikipedia.
Majority of that complexity is not there to make user experience better. I actively use both platforms, and I find that Mastodon provides a better user experience in pretty much every way. The UI is snappier, it has better features for interacting with others, it doesn't restrict the API, and so on.
The main cost of Twitter comes from building tools to monetize and exploit the users of the platform.
Which is precisely my point. Or at least one half of it.