I don't think the designing a new aetherotransformers would be an issue I mean look at SCH and SMN books though I can see it ezing the work load
-Let me be clear I'm not asking them to make new ones just was curious as to why.-

But as for lore reasons as you stated couldn't you say that they could improve on the design as time goes on maybe give you a reason to visit Stephanivien more often IDK
I just don't think its one of its kinda is a good answer...but if its the answer that SE gives us to shut up I'll accept my defeat just think its wierd that box hasn't change