I agree that relying solely on grassroots is bad; larger groups of people are specially hard to coordinate towards common goals. However, as @higgsboson@dubvee.org mentioned there’s more than grassroots backing Solid up. And, even for the Fediverse, it seems that Mastodon caught some positive attention of government entities, like Switzerland.

So, perhaps that’s a bit of wishful thinking, but the teeth might eventually grow, even if they aren’t there from the start.

Regarding your example: it’s tricky for me to talk about USA’s government because I’m not from USA. For me the main issue seems to be the use of winner-take-all representation perpetuating the two-parties system; if that’s correct you’d need more than just a social movement to have a third party, you’d need structural changes. [Don’t trust what I said here, please. From the outside, details are always lost.]