While I agree that ideological reasons will exist in any medium, I think you're proposing an equivalence between the solutions that is made up. Only people with 100% ideological bent will take the actions you suggest; the rest are likely to have many different interests that happen to span purely ideological boundaries, and they will want to remain engaged with those interests such that they avoid finding themselves roped into a place where they only associate with the extreme ideological outliers.
I think you see this in the various ideologically-driven twitterlikes, which have succeeded in only really capturing the extremes and have not made a dent in the mainstream.
Yeah the behavior of instance blocking is an inevitability for these systems. This creates the conditions for those nearer to or already at the extremes to self-select out of the mainstream by choosing to join already radicalized networks.
I think this instance-level blocking helps contain overall radicalization tho, versus the more pernicious radicalizing via mainstream social networks. Basically I don't see them as equivalent in effect, which was the point I was making.
I think you see this in the various ideologically-driven twitterlikes, which have succeeded in only really capturing the extremes and have not made a dent in the mainstream.