The point about Syria is that if Russia and China want to fly over _any_ other country then they must assume that they will encounter VHF radar. And if VHF radar does render stealth technology useless then any include it at all in their designs? Why not slide that scale to zero? Well, probably because there are benefits to stealth and VHF radar is not as easy or cheap as you believe it to be.

You can say that the F35 trades off too much and that a "less stealth" would be better. But that's fundamentally baseless speculation.

Russian and Chinese stealth fighter-interceptors are fundamentally based on air defense and area denial missions. You're exactly right, their main use case is not to fly over enemy countries as much as flying over friendly countries, the ocean, and their own soil.

Stealth is necessary for attacking, but not defending. Further, stealth is only necessary for attacking sophisticated enemies.

Russia and China have no plans to attack the US within the next few decades, so they continue to research, but not to implement. It also doesn't help that US radar is a couple generations ahead as well.

Their current interests involve countries where a fast, hard-hitting plane is just as good as a stealth plane, so they save money at every stage from design to building to cost per hour flown.

Stealth is just as useful for defending and for attacking, and arguably I'd say even more useful for defence because the attacker will never be able to field counter-stealth radars. Stealth when combined with counter-stealth radar on your home turf gives you a huge advantage as you now know a lot more about your enemy than your enemy knows about you. Non stealth aircraft plus counter stealth radar is a lot less of an advantage.

US VHF radar is likely not ahead at all. The US has never fielded a VHF radar, while the USSR and by extension Russia and China have done so since the 70s. The US has UHF radar and is definitely ahead in conventional radar though.

The interests of China and Russia do involve attacking sophisticated opponents such as Japan or Western Europe in order to destroy American infrastructure in a defensive war. It's just that instead of using stealth fighters to do so, they intend on using overwhelming numbers of missiles including hypersonic cruise missiles, of which the US doesn't even have a prototype yet.

These decisions weren't made in the last few years. They are tactical decisions as a matter of doctrine that were made in the late Soviet era. It's not a coincidence that these countries are ahead in counter stealth and ahead in hypersonics but behind in other things, they invested in what was most useful to fit their doctrine.