Is it? You won't use the F-35 to attack other nuclear powers.
If necessary - you will. That's the point - if you can fight with top adversary - most probably you can do it against others. The other way around is a recipe for a disaster.
1.I don't remember nuclear weapons flying left and right over Korea and Vietnam. Neither they flew when mercenaries were caught last year in Syria. 'cause launching strategic nukes is counterproductive to the very first purpose of the military - defence of your own homeland. Launching shiny and flashy things is a game of two(or more).
2.China has a directly stated "no first use" policy. With Russia it is a bit complicated, but you won't put "the very survival of the Russian state into a question" that simply.
3.F-35 is one of the most potent tactical nuclear delivery systems in its own right. So if nuclear war - well, ok.
Btw, while Russia is in a good position(and won't remain idle), China still easily
can loose a nuclear war against the US after a counterforce strike. But - can't win.
So what's the point?
Just see Israel attacking Syria as an example. They're attacking Syria with F-16s.
Stand off munitions, launched from far away, have a lot of restrictions.
1, they carry much weaker warheads for the same mass than bombs.
2, they are much more vulnerable to EW.
3, only man-in-the-loop type systems allow any decisions on place. Not only their sensors are incomarably weak, this very data link is the first and the simplest thing to jam.
4, stand-off munitions(or loyal wingman-type drones), capable of looking for moving and/or new targets on land on their own are still a sci-fi. Expensive future tech.
F-35 is a strike/reconnaissance platform at the same time.
And there are more. Long story short, while stories of 4.5th generation being unviable anymore are just... stories, ability to go in, personal and close is definitely something to strive for.
5th generation fighter-bomber as a core of your force is a real thing, a game-changing vehicle far more important than shiny "i shoot down everyone" air dominance platforms. It's just what it isn't for everyone, all 5th gen force structure is incredibly expensive, blth directly and indirectly. In post-cw the US managed to get it going(and all of their direct allies will get it as a consequence).
Trying to deny this achievement - incredible achievement, - is delusional...