It's not the reality though, it's an unknown. It depends on what the EU and the UK can agree on in their trade negotiations.
The status of the Irish border cannot depend on what the EU and UK may or may not agree in 2, 5 or 20 years time. It needs to be resolved now. Ireland and the UK - the only
governments involved in this situation - have already formally agreed to not returning to any situation that would provoke hostilities. While the Good Friday Agreement does not explicitly prohibit a hard border on Ireland, politically and socially it does.
There is only one reason a hard border on Ireland is even being discussed at all... because the EU require one. But the Good Friday Agreement prohibits it (well, more accurately, it prohibits the effects of one), so the EU's idea is a non-starter.
While the backstop would, initially at least, prevent the need for a hard border, the problem of what to do once the UK leaves the Customs Union and Single Market would just appear again.
The problem is, the EU may never agree to any trade deal that involves the UK leaving the Customs Union, and then the UK is permanently trapped because the backstop has legal force indefinitely, and the only escape route is to agree to whatever the EU demands as part of a trade deal.
Given that I am a Remain voter and a supporter of the UK's membership of the Single Market, even I can see why the backstop is never going to be agreed to.
And frankly, I have a hard time seeing how a hard border can be prevented unless the UK remains in the customs union, or aligns its trade policies to that of the customs union.
You may be surprised at how quickly solutions will present themselves in the event that the UK is forced to leave the EU without a deal...
The fact is though, that the UK has a
right to leave the EU, and thus the Single Market and Customs Union whenever it chooses to. Indeed, no member state or sovereign country can be forced against its will to either join or be forced to remain inside the EU against its will - a fact that is recognised in the EU's own acquis (Article 50 would not exist otherwise).
This being the case, the EU has no legal or moral right to compel the UK to remain under its legal oversight in any way, but that is precisely what the backstop does. On that basis alone, it is not acceptable.
The Irish border issue complicates matters, but it doesn't change two plain facts on the ground... the UK has the right to leave the Customs Union, and Ireland and the UK have agreed to no hard border on the island of Ireland. Those are the ground truths, and thus any permanent solution has to address them both.