I don't think these narratives are ever definitive, but I think this rendition isn't really operative.
>> Social rights advocacy usually goes hand in hand with far-left positions
Maybe to some extent, but mostly I think this is a product of being "in opposition." You get to be more blunt, fiery & vanguard in your rhetoric when you are in opposition. Also party fringes tend to group somewhat.
In any case, "centrism" has been relative to a previously conservative economic outlook and a previously liberal social outlook. A party representing the economic left wing agenda combined with the social conservative social agenda of the 1950s-80s would have bombed in the following decades.
Maybe "elite ideology" has been further right economically and further left socially than the mainstream... but that position tends to be well represented as centrist factions of large parties.
There aren't fringe factions representing these positions because mainstream parties already do. The UK does actually have such a party, the Libdems. They sometimes act as punisher for labour or tory candidates that stray from the middle ground. IE, they get labour votes when the labour candidate is overly radical in rhetoric, and conservative votes if the conservative candidate is overly reactionary. But, any recent British PM would have been at home in the LibDem party.
There's no need for a centrist party that defines political orientation the same way the main factions do. Even in real multiparty systems, this kind of centrism is usually a small, short lived and inconsequential victory. The problem isn't that the ideas aren't popular enough. They're too popular. Both major parties already court these voters.