Exactly how does experience-less vehicles discourage XP farming? If anything, it only encourages me to forego them entirely and get a pair of xenonauts instead who WILL (the sun God willing) grow to become excellent troops. Their not gaining experience isn't a fair trade-off, it's encouragement not to use them altogether.
Besides,
It was a stretch goal - surely they wouldn't propose a stretch goal that went against their design vision?
Seeing my troops grow over time and missions is something I enjoy tremendously, with it comes the pain of losing them and the joy of watching them overcome desperate situations. Beyond the amount of work required (could it really be that much?) I do not see how this would negatively affect the experience - only give those who wish it joy in growth and pain in loss.
Again, any idea whether this might be moddable?