Unfortunately, existing F-15s are wearing out. There is barely enough money to keep them flying, let alone for any "improvements". New F-15 variants could have been built a long time ago, but the money is needed to keep the ghettos pacified.
Su-27s and derivatives are still rolling off the lines.
That may be somewhat true, however, F-15’s are still being produced and are easily the equal of the upgraded SU-27’s (note that I was earlier referencing the 20th century).
The F-22 is in no way inferior to this aircraft, period.
Consider the roles that specific aircraft are intended for.