Since this is a much better deal for Brendan than anyone else, in that he’s currently in one of the two dominant parties and can switch to the other at any time.

I think you’re seeing “a better deal for Brendan” when it’s actually just “not as good a deal for Kevan and Quincunx.” This gives Matt and Viv the option to retain their historic loyalty if they like, or switch to a dominant party, which is a more significant upgrade than me switching from Down to Charm.

I’d certainly support a proposal that allowed Matt and Viv to switch their obscure Parties for dominant ones, if they wanted to. I don’t see a reason to give an unrelated third player an advantage at the same time.