Even if you buy them on gog you don't own them. Download and keep - sure, but you could do that with many games on steam too (also you could download torrent versions which wouldn't be different from buying on gog). The point is about actually keeping these copies alive, properly updated and working, for which these services exist.

So, I think owning a disc is also risky, that means your copy can degrade. Owning games in this context have lost its meaning for me.