The old design will completely go away in the next few days. From a development perspective it's just not feasible to make everything work for two different designs (with many many themes). It's better to spend the time on cool new features.
Actually my point was exactly that everyone seems to have their own favorite thing they miss about the old design. So for me it would make more sense to build a mode (on top of the new codebase) that would mimic the old design instead of implementing fixes one-by-one. Then everyone who liked the old design better could just change one setting that is easy to find and be done instead of playing with bunch of individual settings in different menus to achieve the same thing. I think it would stop a lot of complaints even if it would not be perfect.
I understand that there might be very good reasons why it is not feasible, this is just my 2 cents.