Hmm, I think you're underestimating legacy code. I can name two very large projects in my company that do use jQuery as the main client-side library, as well as numerous other smaller ones (granted, these projects are not SPAs).

Back to the point, jQuery and Mithril are more similar than your analogy tries to make it sound. jQuery does AJAX/CORS and Deferreds and .data() and a whole lot of non-DOM stuff that people use when building their apps. There's some stuff that doesn't map like jQuery animations not having a Mithril counterpart or Mithril routing not having a jQuery-core counterpart, but that's cherrypicking the stuff that is different and saying "look, they're different!", while ignoring the similar, comparable aspects.

Also, I'm not sure how familiar you are w/ underscore or Angular, but both have a templating system and utilities to work with lists of data, so there are definitely points that can be compared. These comparisons are really not at all like comparing something like restangular to hoverIntent, so implying that they're equally imcomparable seems like the dishonest and non-productive argument to me, imho.