I wonder whether this could still pop up at estate sales, or when a retiree is cleaning out their garage.

Not all gear got junked. When I was a teen intern, I got some obsolete Apollos (and 2 logic analyzers and a terminal) from my employer, and other people were also bringing home gear the company "sold" them.

Somewhere, there might well be an industry or university sysadmin or programmer who brought home a box of old QIC tapes, and one of them says "AEGIS" on the label, and it's in a garage/attic.

Also, rumor has it that at one point Boeing physically archived at least one Apollo network, because they apparently take documentation integrity extremely seriously. If that's true, they might have an engineering librarian or someone who could take an interest in making sure any versions of Aegis/Domain they need (and have preserved media for) can run on emulators or something?