What do you suggest they do with the 1000's of British loyalists in NI. Maybe ship them across the water with the DUP to the mainland. 🤣 It is about as feasible as your suggestion.
A huge part of the troubles were loyalist terrified of a united Ireland.
What makes you think they'd agree now.
So what are you suggesting? That the loyalists have their way even if it goes against the wishes of the majority in NI? We can see clearly where the demographics are heading.
Unionists have maintained for decades that NI must stay in the union because that's the preference of the majority. Once the demographic shifts the other way and a majority wants a United Ireland, you can't just change the rules and say "no that doesn't suit us."
You think there would be upheaval if NI voted for a United Ireland. Well imagine the upheaval if such a vote was to be ignored. What kind of message would that send?
If a United Ireland were to happen, I have no doubt that the Irish govt would do everything possible to ease unionist concerns, ensure their identity is recognised and give them proper representation. They would never suffer the kind of discrimination that nationalists had to deal with. Some hardliners may move to the UK and that's their choice, but I imagine the majority of unionists will just get on with their lives.