I can assure you that offline installer you got today from GOG will not work on Windows 20 or whatever OS will be the dominant for PC in 30 or 60 years time.
In fact I used "most" but I can't name one that couldn't be played.
Given the lengths the Windows development team has gone to, to preserve backward compatibility, to the point that there was individual-game-specific workarounds codified in Windows, makes this claim the same as the GP’s, that Steam will change 30-60 years from now.
The cynic in me thinks you’re both right, mind.
Found some other options (fan remake) for now, but probably I need to shell out the $3 for a modern port or run a whole emulated windows95. Probably wine with options would also work? SSI games, Allied General and Pacific General.