I realised it was hypothetical.
But hypotheticals have a habit of turning up, so it can be good to think in advance.
So more thinking:
DLA/AA were basically the same benefit depending on your age when you claimed.
When they started changing to PIP they decided to leave the older group on AA alone- maybe because 'those already on AA would not be round for long'?
Their mistake was to keep accepting new claims for AA rather than making any new claims over SPA an 'Age Limited PIP' without Mobility.
Maybe that was because doing so would have pointed up the age discrimination factor? - I'm double guessing as I go here but that sounds reasonable thinking for the government policy makers.
It's taken them another 7 years to realise that the old system was right, and to start leaving those now on PIP claimants over SPA alone again - but they couldn't admit that mistake hence the 10 year 'light touch' reviews.
I wouldn't be surprised to see new claims for AA disapear altogether in future.
Just make PIP for new claims over SPA 'restricted to DL only' and let those already on AA keep getting it until they are in their coffin.
That would be a much easier change to the legislations. (and ten or twenty years to replace old benefits with new ones is nothing these days).