//…but it [the WA] would have seen the UK formally leave the EU this year,//
In name only, Jim, in name only.
// While I will technically concede this point (actually until 20xx rather than "forever", but for both of us I suppose the end of the century is basically forever), it ignores the practical strain on both EU and UK that a "half-in, half-out" UK would provide. Neither side would want this situation to continue indefinitely, least of all the EU. So yes, technically 2025 has no basis in the content of the WA, but I would still have expected that year (or earlier) to have marked the endpoint of some sort of halfway exit.//
More important (in this respect) than the WA is the “political declaration” (PD) that is expected to be signed at the same time. This sets out the EU’s vision of a permanent future relationship – the one that would have to be settled before the “final exit” you mention above is secured. This is far reaching and its vision effectively ties the UK to the EU and its institutions almost as closely as we are bound now. I’ve copied a passage from another thread where I mentioned this:
-----------
//It [the political declaration] also demands that the UK should align with EU laws and that a future relationship must ensure that a level playing field ensues on State aid, competition, social and employment laws. It also demands fishing quotas for EU boats fishing in UK waters, “Mobility arrangements” (aka free movement), social security co-ordination (so that EU nationals in the UK can draw equal benefits, thus drawing migrants from low paid economies). It proposes that overseeing all of these laws will be, needless to say, the ECJ.//
---------
There is no way a permanent future relationship will be settled unless all of those provisions are met. The idea that the EU will agree to a deal without them is fanciful. It wants full political and legislative control. The UK will be stuffed when it comes to those negotiations because it will be trapped in a customs union (and bound by other conditions of the WA) from which it cannot escape without EU approval and so unable to pursue an independent trading policy. There will be no incentive for the EU to negotiate, only to demand. The WA and PD are disastrous documents for the UK and there is no chance that the EU will acquiesce to anything substantially different.