I'm not opposed to this idea, but I will point out that you don't acquire these quickly. They pile up not because we get a lot, but because we use relatively few.
This leads to a potential downside: folks using up their Injectors by trading them in for relic mats and then can't get new toons from g12 to g13 very easily.
Again, I'm not opposed, and if ppl use up all their injectors, that is a bad decision on their part, but I'm pointing out that it's not without potential problems and CG has to decide if they want to get screamed at by people who use up all their injectors and then get mad they don't have more. (Because you know that will happen. That kind of thing always happens.)
This of course is in addition to the other, more basic problem of CG testing the new economy before they tweak it more.
And, finally, the last consideration is that most of us simply don't need this. With the modern raid structure and the ability to purchase the rewards we need rather than pile up mats we won't use, the bottleneck for r1-r7 is clearly Signal Data, not relic mats. For r10 it's mats, and for r8-9 it's more nuanced, with different players having different primary concerns. But below r8 and indeed at r8 for many players, the bottleneck is Signal Data. Trading in your Injectors for mats won't help that.
To conclude, I'm not opposed personally. It wouldn't help or hurt me. But there are reasons to think that if this happens, it won't happen soon.