If only there was some sort of limited figurehead role for the Monarchy that allowed them to remain in the nominal position of chief parasite but in reality bowed to the whims of a parliamentary democracy. That would be awesome.
As I say, it would be ideal to get rid, but it is important to respect the democratic will which does not see them as a parasite - personally i would get rid of them tomorrow, but I recognise that I am in a minority...
More importantly, we should fix the sovereignty problem left over from history. It has failed to get to the people, stalling at parliament and thus keeping the UK as an oppressive regime. We need to fix this first and foremost, one cannot have a system based on oppression - we need one based on cooperation - everyone else has been moving towards such a system with varying degrees of success, but no one has just kept the old oppressive system, and then used that as a basis for analysis. It is the UK which stands out now as weird, and any analysis of it should be countered with international viewpoints from countries which are more successful such as France, South Korea and the Scandinavian models, etc.
For the vast majority of human history in every part of the globe, this was not the case, yet capitalist social relations transformed things utterly.
You are ascribing permanence to a what is in historical terms a recent state of affairs, and one riddled with inherent contradictions that put it constant permanent crisis and seem likely to lead to eventual collapse. Far from being an "unlikely event", change has been ongoing (in, for example, the extension of the market into previously untouched spheres of human life) and more is certain to come; the question is whether we can make it for the better rather than allowing the dynamic of capitalism to reach its logical conclusion.
It is relatively recent, that is true, which goes back to my comment about the world being a harsh place before, with many people having to eke a living from the land in conditions of terrible poverty.
No doubt there are challenges that need to be faced with this world we have but I do not see it as "likely to lead to eventual collapse" - change will continue but the concept of property and markets to organise the vast people and resources that exist will not, and it would serve the left better if they acknowledged this and moved onto other issues which are more realistic such as tax policy, how to ensure the supply of factors of production is facilitated to keep its costs down, legalising and deregulating so that markets remain competitive etc.
As you allude to, capitalism tends towards control and that has to be countered by government, human rights and a constructive system.
At the moment the so-called 'left' are marginalising themselves by refusing to talk to anyone who doesn't use their version of 'fluffy buzzwords'. It's the same with the religious, they want to talk about 'sin' all the time and point to their book as the answer too. At least they don't abuse as a default position though so maybe it is a comparison which is unfair on the Christians.