I think we are maybe missing the real point here. The US F-15 is not a bad plane, and its age is not an automatic problem. Just look at the report -- Mig-21s can hold their own. The bigger problem, as I understand it, is in the weapon loadout. US planes do not have long range interception missiles. The mythical Phoenix has a range of 100 miles, but it is a very old design, and only the F-14 can carry them.
Our next best weapon is the AMRAAM, which has a range of about 30 miles, but you would rarely want to launch at that range, because that gives the missile little fuel for its terminal engagement phase.
As far as I'm concerned, the US needs off-bore sight missiles (like the Israeli Phalcon (sp?)) and much longer range (ie, 50 mi) missiles. Then, I think we would do much better.