[PLMerite]

"why not just build the F-21 follow-on of the F-14?"

I thought the F-21 was the Israeli IAI Kfir:

[GreyFriar]

I guess I’m thinking of the Grumman working name of “Super Tomcat 21.” It was a revolutionary upgrade of the existing F-14D model.

See — http://www.anft.net/f-14/f14-history-f14x.htm

For the Kafir F-21 — http://en.wikipedia.org/wiki/IAI_Kfir