The British monarchy hasn't always passed down the direct bloodline, but it has almost always passed down a bloodline with a prior claim. The swap between the Stuarts and the Hanovers (as an example) goes back two generations to the sister of Charles I (Elizabeth the Winter Queen) and down through the Brunswick line - and we suddenly end up with a whole bunch of Germans taking over, not speaking much English and generally being very pissy about the entire thing. Likewise the handover from Plantagenet to two separate Houses and then the Tudors is all covered by which brother you prefer. John of Gaunt or Edmund of Langley.

And no... 3/4 is not good enough, and that's kinda the point. I didn't say we couldn't have a POC as a royal... theoretically with a few car crashes we could have Archie Mountbatten-Windsor on the throne at some point... but that King or Queen can at most be mixed race - this differentiates them from an elected President or Prime Minister, who could be any race, up to and including 100% Black rather than mixed.

And yes, they do open buildings and do events etc.... but this doesn't make it less of a racist institution. Every other institution in the UK is governed by a law which states they cannot discriminate on the grounds of race. They are paid public money for providing a service and should be held accountable. I don't have any problems with an elected President (nor do I think it would be better - I just know it would be less racist by design).