That was the problem. No re-assessment date.
You repeated the original mistake. It used to be a once-only declaration from the GP, with no re-assessment ever required.
The problem the assessments were intended to solve is very simple:
A lot of claimants no longer had a valid claim, for two major reasons;
a) Their condition had improved.
b) They had died or left the country, someone else was getting the money.
Thus, everyone who was claiming had to be re-assessed.
So far, everything makes sense.
Every claimant should be re-assessed at intervals - their condition may change and thus need more or less help.
Because nobody had been re-assessed for a decade or more, everybody had to be checked at once.
That was fuck up #1
Then the DWP decided to outsource the thing to a random supplier, instead of to GPs. (Not sure why but I suspect BMA complaints)
That was fuck up #2
Then the supplier fucked it up completely.
The idea was good. The implementation was a complete and total fuck up from top to bottom.