I'll just note that the Su-57s have been unable to do anything about Ukraine's F-16s.
That's btw about immediate loss of efficiency of 4th gen platforms.
If SAMs can create a zone of unacceptable loss chance (which isn't high for 3x strong fleet of the latest aircraft with no immediate replacement in a pipeline), any aircraft can dart to and back from frontline.
Not just F-16s, but anything really (Su-27, Mig-29, Su-25, helicopters even).
With three main lessons out:
1, VKS mostly prepared to defend against NATO, and invested into aircraft parrying leadership's main fear: US airpower personified in stealth.
As a result, it's toothless against lincomparably weaker version of itself.
Great power is only great power if it can attack.
Conclusion: with perspective of hindsight, Russian primary development thrust should've been a more affordable, more attrirable LO strike fighter, i.e. some kind of earlier T-75(Russian F-35).
One may argue that even it's deterrence effect would be higher(no one bothers counting F-35s and F-22s, Su-57s and J-20s separately; higher number is what matters).
Furthermore, in particular Russian case, T-75 can launch all the same effectors anyway.
I.e. for Russia, T-75 is the key enabler they lacked.
Maybe it's possible to try to play with S-70 as forward missile node, but without penetrating fighter I'd argue it's only a partial measure. You need full sensor node forward. In case of S-70 in particular, you also need a flying comissar!
2, main investment for weaker powers should never be high capability, expensive fighters.
Decisive denial assets are SAMs, as they take disproportional effort to root out, and it's always hard to guarantee you really did.
If you have even some air force on top, to interfere with search and destroy type DEAD - you're quite resilient. Powers that can crack this system will probably crack you regardless of your actions, but there's only one such power on planet.
Ukraine got lucky here - not like they invested much in their SAMs, but their huge Soviet fleet was just about enough against VKS.
3, if you drop ambition to cross FLOT at will, you can make do with much cheaper aircraft.
Stealth is a key enabler. At the same time, it's only ever truly matters if you're actually getting illuminated from non-escape distance.
If not, you're just as stealthy at low altitude over your A2AD in practice. You can launch stand off effects just as well.
What matters is, again, dispersal/hardening, service rate, and presence of longer ranged munitions.
And here Ukraine failed spectacularly in prewar era - just absolute minimum of sensible investment would've made their jet fleet capable of biting back in 2022. Ukrainian pilots did what they could, but flying on courage and exchanging turned away Russian planes for lives isn't exactly a good exchange rate. Instead they've only got such aircraft in late 2024.
On the other hand, all European air fleets match the criteria.