And how often does that actually succeed? The American Civil War caused the South to lose out on slavery, at great cost to its economy; while The Troubles didn't induce the UK to give any concessions it wasn't already willing to give beforehand.
Most insurgencies ultimately fail in their aims, and this is already when we're talking about insurgencies against weak governments and civil societies where monopoly of force is close to nonexistent in the first place.
You're saying that the Brits would have agreed to let Northern Ireland vote to join rejoin the rest of Ireland at some point in the future WITHOUT force of arms? I don't believe you. And yes, that was included [0] in the Good Friday Agreement.
And even if most insurgencies fail, "let's not do that because it might provoke an insurgency" is valuable all on its own.
0: https://en.wikipedia.org/wiki/Good_Friday_Agreement#Status_o...
If you look farther back in Ireland's history, it seems like Ireland's independence from UK was achieved mostly by violent resistance. After there was too much violence, England finally decided it wasn't worth it, and came up with an agreement allowing most of the island to become independent, with the exception of a handful of northern counties.
Quite the contrary. The Troubles in Northern Ireland were not a unilateral insurrection, but an emergent response to aggressive incumbent paramilitarism when confronted with demands for greater civil rights for the minority population. Military strategists favored withdrawal of the occupying forces; political hardline thinking won out, but was unable to win a conflict of attrition and eventually settled for power-sharing and de-escalation. At the present Northern Ireland is as likely to see reunification with the rest of the island as continued membership of the UK.