If you're going to do homeland air defence then I guess you need to be able to react to a MiG from Cuba (or Venezuela), a Bear-F toddling down the seaboard, or a non-responding airliner. And you need to do it 24/7 and in all weathers.
By the time you've developed T-X enough to do that, wouldn't it have been cheaper to buy 15 Squadrons of F-16Vs - or Gripens?
And whatever you do to T-X will it have sufficient performance?