Depends on what the PC is used for, isolated systems running Windows CE for example will be around until they stop working and the hardware is replaced.
I have also serviced systems that still run CNC machines on dos based paper punch tape readers. Conversions are possible but costly if it's a milling machine that has been making the same part 3 times a month for the last 20 years it's hard to justify $10,000 for a upgrade sooner or later it'll be scrapped and the part called obsolete or if profitable enough transferred to a newer CNC process and newer multipurpose machine.
Also many like my shop P4 are not network connected have no cd/dvd drive and have the usb ports disabled so infection is unlikely.
