In fact , they have.
The actual top tier for US should be like this.
F-15C MSIP II → F-15C (AN/APG-63(v)3 AESA
F-15E → F-15E (AN/APG 82(v)1 AESA → F-15EX
F/A-18C late → F-14D (APG-71/AIM-54C+ dual plane etc. ECM resistance etc.) → F/A-18E BLOCK I (APG-73) → F/A-18E Block II (APG-79) → F/A-18 BlockIII (2021) (APG-79) - { by pilots, F-14D was better than F/A-18E Block I in …everything , so you can even change what goes where} .
F-16C Block 50 → F-16C ( USAF F-16s don’t use a Block number or letter currently , but sport the equipment of F-16V…call it F-16C Very Late , i don’t know ) .
We don’t really need to go 5th Gens. Too much classifieds , too much wikiguestimation , up to gen 4,5 is perfect and they are operating side by side with gen 5s anyway…
About Su-35S , they can bring it tomorrow , it’s not that they we don’t have other gen 4,5s.
Just have their weapons /radars to an as much equivalent as possible version (version , not state-not everything the same) .
Also,
MiG 29SMT → MiG 29M ->MiG 35S