I think some things were a product of their time, and weren't popular/profitable enough to keep marketing or update or modernize. (whether they didn't maintain peoples' attention, or had issues like the Popeye one you mentioned) Another not-as-popular character that's entering the public domain in 2026, for example, is Betty Boop.
On the other hand, Superman and Batman enter the public domain in 2034 and 2035 respectively, so that should be interesting. Though like Mickey Mouse/Steamboat Willie, I expect that it's only the original version/costume that goes public domain.
DC Comics has done a good job of keeping Superman relevant through time as well. 40s, 50s, 80s, etc. The whole Death of Superman was a big deal back in the early 90s also even though the cartoons I don't think were as popular. You are spot on in that Betty Boop and Popeye like you said are relics from their time and absolutely don't translate - although anyone around in the early 1990s will remember a brief Betty Boop merch resurgance.
On the other hand, Superman and Batman enter the public domain in 2034 and 2035 respectively, so that should be interesting. Though like Mickey Mouse/Steamboat Willie, I expect that it's only the original version/costume that goes public domain.