Honestly, DRM-free is only a minor consideration compared to Steam. I have a pretty huge collection of games on GOG (although not as large as Steam simply because Humble bundles usually offer Steam keys), so it’s not like I don’t approve of the service, and I’d prefer to buy there if I could. However, I don’t have an unlimited supply of funds and making money stretch as far as it can is key (wish I could spend 2x the money without blinking; you’re a lucky one). When the games are the same price (like Divinity: Original Sin 2), considerations may be if the game is a sequel and needs to have the previous game on the same service (or not) to do things like import saves (not typically, so usually not an issue these days), which may get patches first, which might have DLC/expansions first, etc.
I’m not complaining, as I have a choice as a consumer and I’m able to determine what’s the best bang for my buck. And businesses can make a choice on how they want to compete. We all happy.