Getting people off of WhatsApp onto Signal means taking power away from a closed platform that makes billions from manipulating its users, which is already a win. Signal is not only not that, but is in fact a completely open non-profit project, so it's basically impossible for it to turn into that.

Even moving from Fb to Telegram is an improvement in almost all respects and it's sure as hell a lot easier to do than going straight to Matrix/Riot/whatever it is these days. Don't be a purist and let people have their compromises, lest you end up like the "GNU/" part in front of "Linux".

Facebook messenger and Google hangouts actually had federation. So even that doesn't mean a lot.

However I'd also promote federation-first services like Matrix. Only issue with Matrix is the e2e being so clumsy IMO