The F-35 is not just "any platform" it will be the most advanced stealth fighter on this planet once it get's ready and I doubt it will be sold to any country that asks for it. What is more important however is that Turkey is not just "any customer" of F-35 but they are a partner.
Why would the US keep Turkey as a partner in the biggest and most important defence R&D project in it's history (with the possible exception of the Manhattan project) if the US believed that Turkey was about to break out of the relationship with NATO in general and the US in particular?
That's why I think the Pentagon and the White House believes that Turkey will not do that in the short-to-medium term. Longer term, anything can happen of course.
As for NATO; sure it was built to handle the cold war which is now over however NATO has been quite busy redefining it's role since then.
The fact that NATO is currently involved in several operations globally, the fact that no member has left yet, the fact that more countries have expressed a strong interest to join, and the fact that many of it's members are still cutting defence budgets is telling me that NATO has been able to redefine it's role and is therefore viable. Those countries would not be cutting their defence budgets so much if NATO is about to disappear. Also, that in itself is a strong motiviation for most if not all NATO members to stick to the alliance. Many countries in the alliance would need to boost their defence budgets considerably to maintain a credible deterrence without the backing of NATO.
To me the biggest threat to the US and NATO is China -- however China may end up like the USSR. The problem is that we don't know if that will happen in 10 years, 20 years or 100 years... And until that happens China will keep growing and keep building her military capabilities.