Nothing is set in stone. People still get killed because they are on the wrong side of the religious divide, but it has to be born in mind that the world has moved on a long way in the 50 years since the troubles started. Within a few years of the start, both Ireland and this country signed up to membership of an organisation which dictates terms to its members, in theory negating any requirement for a united Ireland. With anyone able to live in any member state, with an outside possibility of political union resulting in a single state, the concept of fighting for a united Ireland rings hollow.
Granted the agreement was signed 20 years ago and has lasted, but that also means a generation has grown up not living in a state of perpetual violence. Paramiltary and criminal activities go hand in hand, but the murder rate in London is now higher than Northern Ireland's, such has been the reduction in violence.
The right of Irish citizens to travel freely between our two countries and live here predates the EU and is intended to continue, so anyone looking to achieve a united Ireland in the context of EU supremacy really needs to look at what they expect in practical terms. With EU law overriding national laws, the nation state is effectively defunct for members. You only have to look at the shower in Westminster to see that there are no natural leaders, such is the level of dependency on Brussels to do our thinking.
The EU says the backstop is their insurance policy, but given the terms, it is most beneficial to them as the one that ensures we will keep the Brussels trough fed for years to come. Anything that means our leaving the EU at their convenience is subservience whichever way you look at it. Rees-Mogg's statement of us as a future vassal state is entirely appropriate.