I swear I've read through this thread but I can't remember if this argument against has been raised. Bringing the tech slot in on the E-wing means all tech upgrades current and future have to be tested against system upgrades, and all future system upgrades have to be tested against tech, and all because of one ship.
Limiting system to Civil War era / Legends ships and tech to TFA and beyond ships saves on a whole lot of work. It also gives the designers to revisit ideas from system upgrades in tech without creating redundant or overlapping things.
I'm not opposed to discounted tech upgrades on E-wings, just posting a reason as to why they might restrict tech to TFA ships onwards.