The difference with replacing the F-111 and the A-10 is that the A-10 was a specialty fighter; it's sole job is close air support and specifically, tank-busting; we have no other plane that is well-suited for this job and none were in the pipeline. Now, the F-111 is a general purpose fighter and, as you said yourself, there are a number of cheaper, well-proven designs out there that could replace it. The F-35 is just the newest in general purpose fighters, so why not buy new so you can not have to worry about it for another 20-30 years...if you were to buy one of those cheaper and proven plane, you're out another several billion only a decade later.

As for US playing catch-up, I think you have your priorities wrong. What is the Cobra manoeuvre (and it's derivatives) honestly good for? You pull that shit and the guy behind you is going to tear your ass to pieces with his cannon before he goes into a dive to avoid what's coming or, more likely, shove a missile up your rear. Yeah, you might get your enemy to overshoot, but with today's air-to-air tactics, that is a highly improbably outcome. Stealth has more uses than Thrust Vectoring does at the moment because you can't hit a target that you can't lock onto unless you do it manually, and that's hard at modern speeds (as if it wasn't hard at WWII speeds).

[/counter anti-US rhetoric]

Related to the F-35:
Now there are posters all over DC calling for voters to vote against spending money on an alternate, more powerful engine for the F-35. We're already cutting down/mothballing our F-22s, so I have to say "WAT!?" The hell are these hippies gonna do when they seem 'dem Chinese flying over their homes?