what the deal DOES do is prove the lie to all those who said the Harrier fleet was old/outdated/unserviceable/unsupportable.
No it doesn't.
Given the budgetry constraints placed upon HM Forces, compared to those that the US military operate under, the last two bits were an issue and were very much inter-linked. All things are not equal, the fact the US have decided they can afford to operate them does not mean they were affordable for the UK to operate.
Also worth bearing in mind is that the USMC's replacement for the Harrier may not even turn up and since STOVL a/c are a major part of USMC operational planning it suddenly becomes "which is cheaper, operating some more Harriers or completely re-writing the entire way the USMC does business"?
On your first two-points though, I'm not sure many have stated that they were 'out-dated' (cue cut and paste quotes) but they most certainly were not old, as stated by some, given the age of other a/c operated by the HM Forces.