There's a reason why social media entities have become walled gardens. Some used to have RSS but removed it. Then they restricted anonymous viewing. Then they motivated users to use the app instead of the website. This isn't just driven by pure evil, it is fueled by the need to keep the lights on.
Running a social media behemoth for the masses costs billions per year. They need to keep online a staggering amount of content forever, develop lots of apps, have teams in every jurisdiction, comply with laws, it's a massive operation. All so that you can use it for free (with ads).
Why would these "hoarders" of social content open up using RSS? It's suicidal.
You cannot escape the monetization question. The Mastodon model moves this cost to individual volunteers, which will not scale to billions of users. The typical instance already has to stop registration after a few thousand users, media content is regularly purged, and instances constantly come and go, so your cohesive "identity" is based on quicksand, as well as subject to inconsistent and arbitrary moderation.
Apart from that, the RSS idea doesn't go beyond the aggregation of reading content. Say that I follow your social feed that way, how do I talk back? Where will my comment be posted? At which service? At which of the various sources that make up your combined feed? Who pays for the hosting? Who moderates? Who gets to see it? Who gets notifications? Which service pays for managing notifications?
What about a like, heart, retweet, all the various types of amplifications? In a decentralized scenario, what does this do, if anything?
What about discovery? How do I discover people to follow and relevant content? Supplying the current effortless experience?
How does blocking and privacy work in an open feed? I'm subscribed to your feed but don't like a particular person commenting on it. So I block him. Who maintains/stores this block?
How do ads work? How does verification work? How do creator fees work? How do subscriptions and newsletter work?
All of these things are standard features of modern social networks that people have to expect. Reading content is only a tiny part of the total social media experience.