Northern Ireland voted to remain in the European Union and benefits greatly from its membership. As a rural economy, it relies more heavily on EU subsidies than England. The issue of the Irish border, meanwhile, has been the most difficult sticking point in Brexit negotiations. Many residents of both Northern Ireland and the Republic of Ireland currently cross the border every day for work and leisure and do not wish to see a return to a militarised, "hard" border, both for the inconvenience and over fears this will trigger violence in Northern Ireland again.
The potential for violence caused by a Brexit-induced hard border has arguably already been visible. Violence erupted in Derry/Londonderry in March, when journalist Lyra McKee was killed in riots. The obvious solution to this problem is to remove the border entirely. This would allow for seamless interactions between North and South even after Brexit as Northern Ireland would remain in the European Union, as its population desired. It would also allow the rest of the UK to finally leave the European Union without the need to finalise a course of action for Northern Ireland and the Irish border, thereby fulfilling the result of the 2016 Brexit referendum.
A United Ireland would merely recreate the exact same issues that may have arisen after Brexit. While Northern Ireland did vote to remain in the EU, a large portion - especially among Unionists - voted to leave and would not accept being forced to remain in the EU after believing they had won the referendum. While cross-border Irish interactions may be easier, cross border interactions between Northern Ireland and Great Britain would be more difficult. Many in Northern Ireland do business or have family in Great Britain, and a United Ireland after Brexit would suddenly present the same type of border problems that a North-South hard border would create. As for a hard border causing violence, a United Ireland would surely be even more likely to cause violence and potentially restart The Troubles.
[P1] Brexit and Northern Ireland's existence create problems than cannot be reconciled [P2] Therefore, the only solution is to unite Ireland to allow Brexit to occur without the adverse effects it would have on Northern Ireland
Rejecting the premises
[Rejecting P2] Creating a united Ireland would not be sufficient to safeguard against violence.