You'll have a much easier time finding a decent JavaScript dev who can quickly get up to speed with Knockout (or Vue/Mithril/etc)... compared to finding someone

You'll have a much easier time finding a decent JavaScript dev who can quickly get up to speed with Knockout (or Vue/Mithril/etc)... compared to finding someone with "3 years experience in Mega Framework X", where X is only a couple of months old. Now, if you might have an argument if you mean recruiting in terms of, "What sexy thing will help me attract rock star 10X JavaScript devs, who haven't used X professionally yet but are dying to do so?". However, the problem with that argument is that people who come to you for the latest trendy edge will leave if you don't chase the next trend every six months. In fact, even today they're already more interested in the React shop next door. Frankly, if you're willing to be a bit less sexy with your front-end tech stack, you can hire more stable and affordable developers and probably be better off altogether.