Theoclymenus: It's a really good question, though computer technology being as it is is I doubt whether anyone will be able to play ANY of their games - GOG or non-GOG, physical or digital - in 20 years' time, regardless of GOG's stance or any legislation. We may or may not still have the legal right to play our GOGs, but in practice we are unlikely to still be able to do so simply because of backwards compatibility issues - even if GOG still exists then. The computer games industry is, sadly, a throwaway industry because the ARTWORKS which many computer games are can only be accessed via a constantly changing medium : the computer. It's very sad, to be honest.
Not everybody trashes old PCs, most of those still work fine.
Heck, even my two old C64s still work perfectly. Those are sold by now though, since I don't need them anymore thanks to emulation, just like I don't need Dos-PCs anymore thanks to DosBox.