Physical games or a model like Gog ensure you will be able to play the games in the future. Digital games where you can't transfer the offline installer to any platform you want is trading convenience today for a guarantee you will buy or pirate it in the future.
Also, fundamentally the issue of copyright is a government enforced artificial monopoly. It's ultimately up to to the governments whether you can "own" any digital media in the future. Hypothetically, the governments could come to take your physical media, and the only thing you can do to defend your rights is with arms, hence the US's fixation on their Second Amendment.
They could be acquired or go under just as well and I doubt that most people are keeping backups of all the installers.
But unlike a disc or cartridge you can have it backed up in multiple places, more similar to family photos.
For those that buy games on Gog, what has been your strategy for backing up the installers? It seems they only make the last few patches available for download. Do you download every patch, or just the installers?