"Were the F-35 likely to be adequate in the mid-2030s and beyond, no-one would be spending money developing new sixth gen platforms and systems of systems. "
Not so - even LM would be spending cash (preferably taxpayers) to keep their design and development teams busy
And there are always armed forces who will specify a new threat that requires new and better and more expensive kit - if only to create jobs in planning. mission offices and contractor liaison. The whole system is set up to keep the production of new stuff running. It wasn't called the Military-Industrial Complex by Eisenhower for no reason
Look at the cash being expended on the Tempest - when the RAF can't even afford to buy the F-35 in penny packets