You're not comparing like with like.
What I'm sure many of the posters who made remarks with reference to the Harrier's servicability were speaking about is it's serviciability within the given constraints of the MoD budget.
Lets not beat about the bush here, the MoD has never taken through-life and spares support to a/c seriously, indeed the lack of support is seemingly excused using the "well something new will be along soon" excuse, though such avoidance leads to situations like that of the Bucc and F3 fleets, whereby they were extremely long in the tooth due to waiting for the next best thing to replace them.
Now I'm well aware that the Harrier was rather expensively upgraded not too long before they were retired but the spares situation didn't improve. Lovely upgraded a/c need spares as much as knackered old ones do and given previous by the MoD is it any wonder doubt was cast over the practicality of operating the Harrier going forward. The continious operations of the Harrier in Afghanistan for so long exacerbated the issue, meaning that once they were replaced by the Tornado the writing was on the wall.
Now why would the US wants them? Well unlike the UK they are a country that has a long history of wringing all it can out of an airframe, the expected OOS date of the B-52 and the upgrade program its been through is a good example. The existance of lines upon lines of a/c parked in the desert also gives a clue to the US mentality towards spares support and 'what if?'
Bottom line, what the US DoD and UK MoD considers 'supportable' and are often two very different things.