For the first years GOG had a no-deep-sale policy actually, with them often saying in interviews they don't want to devalue games. They had the summer and winter sales (or was it just the winter one at first?) where nearly all the catalog was 50% off and a few games maybe a bit more and a midweek "gem" every other week, usually at 60% off, but sometimes could be just 40-50%. Then came the insomnia concept to answer Steam's discounts, with deeper discounts but limited quantities. Then weekend sales and big sales every season with many games at 75+% off and now weekday sales too, so there are always sales going on. Steam took the market there and they followed because nobody bought stuff except either on release or a deep sale anymore.
But the interesting thing is that this spreads out sales more. I mean, the very concept of backlog. How frequently did you heard of that several years ago? Now everybody has one to last them a few lifetimes it seems, and that's because instead of spending a certain amount of a few games they were going to play, now they spend a similar amount, or possibly a tad more, but at a few bucks a game that gets spread out, leaving them with loads of games they're not playing, letting stores keep the income level in spite of drastically dropping profit per sale, but also spreading out the revenue among developers and publishers more, as people are far far less selective. Which also means that instead of making one good game, now the idea is to have something out there and you'll probably get a few bucks from a bunch of people.
Still, I do think that for any game that's 10+ years old, and even 5+ for smaller/simpler ones or those no longer actively maintained and which didn't get significant expansions over the past 2-3 years, a few bucks is the only right price, so 10 for something over 20 years old is ridiculous. With new ones this "discount culture" is rather unsustainable though.