PAK FA is a sound concept, fast and maneuverable . There appear to be some problems and delays with it, but even if it fails, Russians built only few prototypes so it would not cost them arm and leg .
J-20 is also sound concept. It doesn't have OVT (yet) but it has canards . Again, even if this project fails, China built only few prototypes so it would not cost them too much .
On the other hand, we already have over 100 F-35 that would be obsolete according to Lt.Gen. Bogdan when that plane finally becomes operational . F-35 program is now too big too fail, with billions already spent. No one in Washington could now pull the plug on F-35 . That means, like it or not, we will have hundreds of F-35s in service . And that also means that F-35 would suck the air from other planes, trying to take over their roles even if it isn't suited for them .
What would happen with F-15 ? Plane is no longer in production, barring some unexpected foreign order but USAF definitely would not get new examples . Sure, it could be refurbished and upgraded to certain extent, but let us remind ourselves that as we speak Russians and Chinese are producing brand new Flankers .Upgraded F-15 could hold its own against them, but dominate them - no . Brand new version of F-15, with reduced frontal RCS, lighter and with more powerful engines would be a very interesting thing, cheaper and in some aspects certainly better then F-35 . But this would not happen, so we would have to live with existing upgraded F-15 . Don't get me wrong, plane is still better then most of existing fighters but it's age is beginning to show . It could serve with distinction for another 5-10 years, but after that ...