These architectures were for were high end machines at the time, universities and government agencies would indeed been the only ones buying these, and they probably migrated to commodity hardware long ago; if migrating to a PC-compatible architecture had been off the table for them, so had been keeping these things running after their EOL.

(In my second-tier university at my developing country, the Sun workstation hadn’t been turned on in years by the late 2000s, and the the minicomputer they bought in the 1980s was furniture at the school)

Edit: As for big businesses, they have support plans from IBM or HP for their mainframes, nothing relevant to Debian.