My understanding of the imperial starfighter situation-
-"Well, we need to be able to mass-deploy fighters across most of the known galaxy, so we need really cheap fighters. Let's produce TIE-Fighters - no shields, no hyperdrive, so they're dirt cheap. The many pilots who die will be doing their duty. ...And let's also produce the somewhat better Tie Interceptor and Tie Bomber in more specialized roles."
-"But, you know, sometimes you really need a starfighter with shields and hyperdrives. So let's also produce some of those gunboats the Cygnus corporation designed. And a personalized, modified TIE for Lord Vader."
-"Oh jeez, the fact that our pilots tend to die in one of their first engagements means they tend to lack experience and can't really stand up to Rebel pilots. Maybe we do need a better, elite class of starfighter. Let's design the TIE Advanced/TIE Avenger based on Vader's ship, and the even better TIE Defender."
Interestingly, Thrawn gets involved in the development of the new generation of TIEs in both Legends and the new canon: In Legends, he participates in the program that develops them along with Admiral Zaarin (in the excellent TIE Fighter PC game). In the new canon, he's the one going "forget that stupid Death Star project, we'll be able to crush the Rebels if we mass-produce superior TIEs" in Rebels.