Surely their "deaths" had a greater part to play in the show choosing to retire their characters rather than a profound respect for the craft. Had Hartman and Wallace quit, I expect they would have been replaced in a matter of seconds. Shearer is a different kettle of fish due to his personal celebrity combined with how many characters he did but at the end of the day, get a good Flanders voice etc and the show keeps on churning out eps.
Deaths in quotation marks? Anyway, no doubt the fact that they died rather than quit was a factor. My point was that the actor brings something to the role that is not easily replaceable and isn't reducible to the sound of a voice. As Matt Groening
said at the time: "My guess is that none of the writers will be inspired to write for those characters without Phil Hartman. It's possible, but I don't think so[.]"