The force for centralization is that for social networks it simply is the natural topography. People are drawn to where everybody else is, so something being central is a main attractor, even if we disregard the ambitions for reach. Spam is a secondary factor at best.

While Spam is a problem and affects decentralized systems more easily (if they have a critical number of users), the cost of client attestation is just too high.

I am perfectly happy if the web and stays open and a lot of people go into the app space and stay there. I am happy for facebook and don't think I am missing out on the web. I don't use any apps for social media and exclusively use browsers. I wouldn't want a second app space on the web at all because the mobile environment is an ugly abomination of software crap.

If we have a form of RA, it will get worse for users and developers alike. It will be a far worse hassle than killing a bit of spam and we give the wrong players too much power.