I am curious how the long tail and changing of media consumption habits has devalued many of these characters. Mickey is Mickey and there's an entire company built on it of course, but my kids have literally no clue who Popeye is. They have never seen a Popeye cartoon and probably never will. When I was younger we didn't have nearly as much choice (2-3 channels with limited times for cartoons) - rather than Netflix or Youtube which effectively offers unlimited/fragmented options.
Additionally the main plot line of Popeye is effectively Popeye protecting Olive Oil from being assaulted by Bluto - not exactly modern cartoon material.
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.
Additionally the main plot line of Popeye is effectively Popeye protecting Olive Oil from being assaulted by Bluto - not exactly modern cartoon material.