Well, Pdox shat the bed with the reserve mechanic already in HoI3 so I'm not surprised that they don't want to re-open that can of worms again. IIRC, it's missing from HoI4 too.
But yeah, it basically looks like some players want to be able to have more ships than they can maintain and/or reduce MSP consumption of their fleet in general. Since Steve has repeatedly said that the mineral acquisition part and the fleet logistics management part are important & elementary parts of the game, I don't see him adding a mothball mechanic that would simply let players ignore maintenance - and you can already turn it off if you don't want it all, of course. And I also agree with El Pip that if the cons of mothballing are severe enough so that it isn't a simple MSP circumvention tool, then it probably isn't a viable strategy in Aurora warfare where years of shipbuilding can be lost in few minutes. The sort of desperate delay action from JP to JP while waiting for the Reserve Fleet to get activated and rushed to the front, that we're so familiar from Weber's books, just isn't really viable in Aurora since almost nobody gets empires large enough for that to be possible and there's no NPR that has enough ships and brains to pursue a chase campaign like that either.
Which means that Steve would need to code a complicated mechanic that's only really useful for few RP situations and for PvP campaigns, both of which are pretty rare in my understanding.
I'm surprised about the hand-wringing over balance for this issue; mothballing fleets is a feature you are right to expect in a game such as this and compared to the other ways you can cheese the game I don't think its terribly offensive.
I don't agree that it's a feature that people expect in a game such as this - I can't off the top of my head recall any 4X or grand strategy or space war game where it's possible or at least an integral part of the game. And sure, we are not forced to use such a mechanic but alternatively, you could simulate it by using SM mode to create a dozen massive maintenance stations at Triton and parking your 'mothballed' ships there, and every now and then use SM to add more MSP to them.