I agree they have eroded the value of plus tickets over the years since inception, but at least C tickets can be changed anywhere, including at the gate (which isn't the case for diamond perk, at least not officially).
Aegean has implemented the same perk as part of their more expensive ticket type and it is very nice, but I'm torn. At least if it is an either/or - It is such a great diamond-perk to be able to change departure of any ticket type I wouldn't want to lose that.

At the EBD event in Copenhagen last week, Anko said they "soon" will have the perk implemented in the app. He gave the impression it was coming in a month or two - an EB officer I spoke to later said she was less optimistic about the timeframe because initially "after summer" was the estimate and so now she guessed "early 2024".
The interesting thing was that they were well aware that the requirement of having to go to the airport to make the change is very limiting. They had discussed if they could let agents in call center do the changes too, but decided against that as that would mean having call center agents accessing the departure control systems which they have no experience with.

Another feedback I gave was that not all staff/outstations knows the policy or how to handle it. The EB staffer immediately took it to heart and promised the outstations I had mention would get training.
There are outstations that "for technical reasons" are not able to make changes (Salzburg (SZG), Dubrovnik (DBV), Pula (PUY), Zadar (ZAD), Split (SPU), Tivat (TIV) ) but for all others check-in agent should be able to do it.

For connections at ARN, the T5 service desk is tremendous. They got me on an earlier flight that was already boarding (some other diamond was sad I took her seat block). I doubt the app implementation will be as flexible? I reckon there must be cut-off times or people will change to connection they can't make in the end.