I still think the less romantic version is that Matt Robinson was about to leave the series anyway, and the character left with him.
I'm pretty sure that Matt did continue performing his voice for at least a few years after he stopped playing Gordon. Roosevelt did have a speaking part during Hal Miller's first scene as Gordon, and Roscoe Orman did perform Hard Head Henry Harris (though Harris' voice does sound a bit more like the deeper voice Roosevelt had in his first few appearances... Is Sesame Street Unpaved the only source for Orman voicing that character? Was he credited as such on that Christmas album where Hard Head Henry Harris sang a line in The Twelve Days of Christmas?). Hulu cites one Roosevelt Franklin sketch as being from season 6, and the 40th anniversary DVD cites another as being from season 4 (and there are no known episodes from the first three seasons that feature any classroom sketches).
I sort of wonder, if a character is taken off the show, particularly after being on for such a short time, if Sesame Workshop tries not to show old segments with the characters too much, for fear of them creating a fan base and demand for more new segments with those characters. Though some did continue to appear in repeats for years... It seems there's not many Lefty segments, and I'd be surprised if there were any after the 1970s ended, but I recall seeing them a lot as a child (the last time I remember seeing one on the show was in season 29). And Harvey Kneeslapper segments were shown until season 32 (though it seems like there'd be periods when the character wasn't shown, and then periods when segments with the character were shown a lot... I recall seeing quite a few Harvey Kneeslapper segments in seasons 25-26, and I know a few were shown in season 32). And I think segments with Herbert Birdsfoot were shown very occasionally up until at least season 25 (I remember seeing the M/W one that season).
Though when Jim Henson died it was originally decided not to recast Ernie or Guy Smiley, but those characters had plenty of segments to reuse (and there were plenty of Kermit segments to reuse, in case Steve Whitmire never performed on the show). And the case is similar with Richard Hunt, though I feel there weren't many inserts with Placido Flamingo or Don Music (and segments with them were shown up until the late-1990s). Though there are also a lot of recurring characters who seemingly weren't in a lot of sketches but their sketches were repeated for many years. Characters like Mr. Chatterly, Bip Bipadotta, Little Jerry and the Monotones, Chrissy and the Alphabeats, Sinister Sam (though there's not much you can do with a character with a reputation for being mean who always ends up being nice), Bad Bart (ditto), or Colambo. In fact it also seems like Simon Soundman, the Twiddlebugs, and the Martians only did a few inserts a year (or every few years), though they all ended up with a fairly big back catelogue of inserts. I also feel like there weren't that many Sherlock Hemlock inserts. It seems to me that most of his appearances were in season two, and then he showed up for new segments every few years. I don't know of any Sherlock appearances during the 1980s (aside from background appearances in a few movies and specials, mostly Muppet productions with Sesame Street characters), and then he had a regular role again with the Mysterious Theater segments (which only had four known installments).
And then there are characters who were only on the show in new material for a very short amount of time, and it seems inserts with them have very scarcely been repeated (in English) after they were departed. It's very rare to find English-language copies of segments with Professor Hastings, Aristotle (who was probably more of a street scenes character), Deena and Pearl, Leslie Mostly, Warren Wolf, Leo the Party Monster, or Dexter (again, he was probably more of a street stories character, and considering how complicated he was to perform, there probably weren't many segments with him, whether they were street scenes or inserts).