Then that's not an 'exit'. The EU must treat the UK like any other country (say, Canada or Korea), otherwise all the EU countries will exit as they can get better terms than just staying in the EU. CPG Grey has a great explainer on the impossibility that the UK is in:

https://www.youtube.com/watch?v=J1Yv24cM2os

If you are unaware of the issues with ROI and NI, I'll say that a bit of education is really needed. The history there is very complicated and can't really be boiled down here on HN.

That said, the threat of renewed violence on Ireland is absolutely real and very possible. During The Troubles (lasting ~35 years) there were ~50k casualties in total. The peace is fragile and renewed violence could very possibly result in much higher casualty numbers.

Lives are quite literally on the line.

EDIT: Sometimes pictures are better than words. We can talk about 50k casualties, but seeing some of them is a lot more 'real', at least to me. If you have the inclination, this image gallery does an alright job showing the long and very bloody history of ROI-UK relations during The Troubles: https://allthatsinteresting.com/the-troubles#31

Massive VAT fraud. With no border check you say whatever you want for the value of the goods. Leads to situations where merchants can pocket most of the VAT.

You could have that by agreeing on a free trade deal between the UK and EU, which is what May's Withdrawal Agreemenent wants to do.

The problem is that by using a trade deal the status quo the UK is effectively losing its voice in the EU parliament without gainign anything in exchange. This is unsatisfactory to both the leave and remain camps of the UK parliament.