In a military context, the main technological gaps relating to jet engines are supercruise and maintenance hours. Both are potentially quite relevant in a ‘local’ conflict.
Supercruise also matters a bit less when the distance to combat is shorter. Less fuel expended on "getting there" is more fuel for "on station". Plus, assuming more-or-less unlimited supply of machines and pilots means more flight hours on station.
So while the engines play a part in a hypothetical conflict, supply lines (and the length thereof) play (I think) a larger part.
Maintenance hours are sure to matter in a real war where equipment is getting destroyed and supply lines are being disrupted.
The F-35, which was optimized for congressional district employment, has an effective 25% readiness rate (another 25% can fly but not fight), which is half of the 40 year old F-15C and 1/3 the newer F-15s.