Necessity is the mother of invention. The Terran-Vasudan war had been ongoing for fourteen years with neither side gaining a decisive edge during that time (that we know of). It's not out of line to think that both sides had settled into a more-or-less comfortable stalemate while developing new weapons and fighters in preparation for an ultimate showdown of ultimate destiny. The GTW Avenger had presumably been in development for some time before the arrival of the Shivans. And when the Shivans did come, the stakes changed from simply "winning a war" to "survival of two sentient species," which I think is motivation enough to speed up research and development of projects already on the drawing board. Political, bureaucratic, and economic considerations are meaningless if one is not alive to deal with them.