I think ultimately it will slowly but surely become "Brexit without Brexit". The UK will rejoin the customs union, which in turn will necessitate rejoining various agreements with the EU like Schengen, etc. Economic integration will resume, just with the right colour of passports.
That's a worse position to be in than actually being in the EU (most of the obligations without any say in the decisions), and it'll probably take a decade or more and a lot of pain to get there, but I'd imagine it's the least-bad option if rejoining the EU is simply a bridge too far for many.
The EU would be mental to let the UK back in to any formal arrangements like the CU or SM until the Tory party changes radically. Sit in their chair for a moment.
Let's say the next government wanted to take us back. The EU would have to take into consideration the fact that a change in government back to the Tories would just mean our leaving again. They won't take that risk.
Until the EU sees a broadly pro EU sentiment on both sides of the HoC we're out.
Until we see consensus across the HoC I see only movement towards working with the EU a little more co-operatively rather than hard, formal ties.
We're out till my kids are my age is my prediction.
Vin