I heard that the 20.3 release is the last release to have functionality update in Classic interface. Going forward, things will only update in NWE (one of the reason I've lied to my people and said they have to all be in NWE by beginning of October ;) ).
If you have specific feedback, I would definitely share that with Workfront Support as they can work on those items to fix them. Like for us, we have too many people still using the old Resources Grids so let Workfront Product know that the Workload Balancer has to be working and able to be on a dashboard before we could really start transitioning and Vazgen and team have been working hard this year to get that for us.
No tool can be around forever without updating. Could you imagine still using the old interface for Facebook? HAHAHA! I have feeling some of this is them just not wanting to change and they are using font size (which can be fixed by zoom BTW, as I have users having issues with font size) instead of just dealing with truth.
I would recommend talking with your executives and tell them the reality, yes there will be some productivity issues (like one I've said is that the issue description is no longer in header of the issue so you have to keep clicking back and forth between Issue Details and Updates when answering items in a request). But is that productivity issue worth having potentially out of dated and unsupported technology in the future? And have them decide. I promised to continue to let Product know this would be a big help to us and does annoy our users, but now the the team decided no we need to move into the future. That feels better than it just being you who made that decision and everyone annoyed with you. ;)
BTW - to let you know, when we first showed NWE to people and moved some people over, almost everyone hated it. Several months later, now I have almost 9,000 users using it and everyone is fine. Some feedback here and there (which I share with Product), but otherwise it is working.