MS DOS

use cases in 2020’s :

Many video slot machines in casinos still run DOS, the newer ones - FreeDOS. I’ve seen some with Linux, but not many.
Apparently the software for a casino game must go through a very expensive certification process, so rewriting it is not really an option

still some ancient fire alarm control board
which can only be programmed using special software on MS-DOS, which we run on a Tandy 1100HD

In industrial work environment , most machines date to the late 1980’s and costed a fortune.
They would be too expensive to replace (considering each is over 100,000 dollars and there is a whole ware house of steel CNC machines) and still all operate effectively when kept under regular scheduled maintenance and repair. I am in the electrical engineering department and see myself working on these machines quite often and have noticed that all of them have controller systems running MS DOS 6.22 or Windows 3.11 (over DOS). We even have original hardware to go with that. For instance, one of the recent machines I had to repair had a Toshiba t3100e/40 computer, which really took me by surprise. If it works though, why bother replacing? I will admit we junked two tractor-trailers worth of Apple iie’s (actually piling them into trailers there were that many) but the Apple systems proved noneffective and most began to fail quite miserably. Those systems had to be upgraded.