The F-35 has never been one of the "most commercially successful airframes". 670 sold is actually a pretty low number, considering its supposedly multi-role capabilities and its rare VTOL feature.

The sales pipeline started being agreed when the plane was still a concept, and it's almost exclusively a feature of American foreign policy: the US government worked hard, over almost 40 years, to ensure a significant part of design and production costs (and risks resulting from such) would be absorbed by allies through guaranteed sales. NATO countries have been under huge continuous and sustained pressure to buy it and to standardize their systems around it. Quite a few European governments found themselves struggling to publicly justify their choices in this matter, because in the end it mostly boiled down to "the Americans told us to buy it or else". The F-35 project simply would not be allowed to fail in the market.

As far as I know, no other weapon ever enjoyed such massive and forceful support by so many US administrations throughout the decades.

> the US government worked hard, over almost 40 years, to ensure a significant part of design and production costs (and risks resulting from such) would be absorbed by allies through guaranteed sales. NATO countries have been under huge continuous and sustained pressure to buy it and to standardize their systems around it. Quite a few European governments found themselves struggling to publicly justify their choices in this matter, because in the end it mostly boiled down to "the Americans told us to buy it or else".

So, it’s SAP but for national defence