I guess this is really the key, at least from where I stand. Ditching FPTP should make it possible to actual-vote your conscience rather than just protest-vote your conscience, helping out minor parties; and trying to build more robust coalitions beyond the confines of the two-party duopoly could give a voice to the smaller parties marginalized by FPTP (i.e. the ones that should have the biggest stake in ditching it for a better system).

I don’t pretend to know which end it’s better to attack from, but it makes sense to me to push for both: supporting third parties while pushing for reform. If nothing else, I’d think an exodus from the major parties could help spark more conversations about alternative voting systems and move that discussion more into the mainstream. So while I agree with pretty much everything you said in the body of the post, I was more taking issue with the implication from the title that voting for a third party is a bit of a wasted effort.