It's nothing like that simple. What you are not being told is that the apparent trend on tribal head count numbers last time around was mainly due to a relatively large influx of Polish and Portuguese economic migrants through being in the EU rather than what was happening amongst people born and raised in NI. That hasn't been continuing to happen at the same rate.
Also worth noting that there is a growing group that identifies as neither RC or Protestant because they realise there is no sky fairy. Until recently they have tended to be from a Protestant rather than an RC background, but that's changing so it's quite possible that there will never be 50%+ that identify as Roman Catholic and that both religious groups will slowly recede in the years ahead as rational thought becomes the name of the day.
If things do settle down to something reasonable where the border is concerned, UI could quickly fade as a pressing issue. The bizarre thing about what has been happening is that the DUP have appeared to be doing their best to engineer the scenario where that's definitely not the case.