IMO, KFX is still a significant leap over the FA-50, you get a twin-engine fighter with indigenous AESA radar, US and EU armaments (including BVR) and probably what looks like lower RCS compared to other 4.5 gen fighters. Meanwhile, despite the FA-50 being upgraded to Block 20+ with BVR capability, I don't think its going to have the same future upgrade potential and growth as the KFX.
But here's the thing, you really shouldn't substitute the F-35 with the KFX. The KFX is basically going to be late in this case, in 2030 while everyone is starting to fly 5th gens, the KFX which is a 4.5 gen fighter is probably going to only reach operational capability by then and we don't know when the supposedly 5th gen Block 2 or even Block 3 KFX is going to be developed considering manufacturing for Block 1 is only going start in 2026 if nothing goes wrong. Meanwhile, in the mid-2030's, you'll see European manufacturers are going to start to roll out 6th gen prototypes and developments like BAE's Tempest and Airbus's FCAS. So in this case KFX is going to be quite late in its development compared to others. So depending solely on KFX isn't only going to undermine our air power/sortie generation but also our overall capability in general when compared to others. Just look at ROKAF, they're still operating F-4's and F-5's simply because of the need to be able to generate a certain number of sorties and to complement their far more advanced KF-16's, F-15K's and F-35's. Meanwhile, in ROKAF, the KFX is meant to replace the F-4 and F-5 which is the lower tier fighter in their fleet and complement KF-16, F-35's and F-15's, so its obvious even ROKAF themselves doesn't see the KFX as a potential 5th gen or F-35 substitute and neither should we.
IMO, I support continuing KFX as a potential twin-engine for the 2030's, but not by sacrificing more necessary and needed acquisitions like the F-16V's which is our reliable workhorse for over 30 years and is required for our airpower generation as well as to face more pressing adversary and threats we're seeing up north.
So in this case, I wouldn't agree with people who only want solely depend on the KFX especially with its uncertainty coming from our side and its lack of capability for its targeted time frame as well as its lack of maturity which will take time. National prestige isn't worth it if you're sacrificing combat effectiveness and ability to generate power at the end of the day.