What we can't have is a soft border and WTO.
Why not?? The WTO are actively encouraging frictionless borders.
We could and should be at the forefront of developing the protocols and technologies required to make this work and then could export this expertise worldwide.
That's probably magical thinking for most remainer types though.
But none of the infrastructure is currently in place....
There is already infrastructure at the border as there are cameras there, so all this talk of no infrastructure is nonsense. If you mean physical barriers however, that just isn't going to happen as all parties involved have pledged not to erect a hard border, even in a
No Deal scenario. As said earlier there are WTO provisions for contentious borders where WTO members can take actions not taken at other borders for security reasons.
....the implementation phase is currently planned to be 3 ish years which isn't a lot of time for any Govt high tech scheme.
HMRC have already said that they can adapt existing systems for the Irish border and can have it operational by the end of the planned transition period (December 2020), which looks likely to be extended anyway to December 2022.
That isn't what happens. What happens is that some third country member of the WTO complains to the WTO about a second country getting more favourable treatment. Whilst the WTO decides on the merits of the case (takes months/years), that third country typically imposes punitive tariffs on some import to their country from the offending country/countries. If that third country is the USA complaining about us giving unfair treatment to Eire imports, then there are plenty of things they can penalise that will really hurt. That's basically what happened between Canada and the USA over Bombardier.
OK that's a fair point, but just an example of the normal rough and tumble of international trade I'd imagine. Compare that with getting stuck in a Customs Union with a (questionably friendly) foreign power, where there is no legal right of exit without that foreign powers agreement, and no say in how that customs union is operated, in terms of trading arrangements with third countries, the tariffs we'd have to charge and market access we'd have to give.