Snafu,
The answer to your question is (as we all know) that any system's capability against a threat will suffer due to long gestation as that threat either develops or changes completely. Somewhere in this thread or a similar one someone posed the question as to the F-35s stealth and in what part of the spectrum it was stealthy. i.e. one sensor may not see it but another possibly could.
The other thing to consider is that requirements change as well as threats. If you buy a machine for a full-up state-on-state scrap that never happens you will 'waste' a lot of money and end up with a system that isn't as good at other stuff as legacy systems. Thinking COIN specifically. However, defence as always been about insurance policies, so you might not want to be without the full-up capability.
In answer to your question about what happens to the non-players...well, if you don't make the grade which is usually given in some form of theatre specific orders, you don't play. An example might be that if a GW1 scenario (or any for that matter - but you get the point) was moved to the 2020 timescale the ACC could well direct that only LO platforms would go north of the border. So a country that doesn't buy F-35 (or F-22 I suppose) might not even make the ATO.