Did designer defections leave the Empire stuck with the TIE?

Did the designers of the Empire's intended replacement fighters defect to the Rebellion, leaving it stuck with the TIE?

That does not fully explain the failure, because the rebel fighter families were already in development and a galaxy-scale state should have had other designers able to improve the TIE.

A viewer suggests that the A-, B-, X-, and Y-wing design teams defected before they could replace the TIE. Those fighter families were already developments in production rather than one unrealized Imperial succession plan. More importantly, losing particular designers should not leave an enormous state unable to improve its principal fighter. The continuity explanation raises a defence-industrial problem about maintaining multiple sources of technical talent. A resilient procurement system cannot depend on one small group of engineers.