Shocker650: When you are not a fanboy of one and a hater of the other, you can play all the games you are interested in. At the end of the day they are all free, and you don't even have to buy a seperate platform. As far as game preservation goes, it's not Valve's fault that publishers like Rockstar, Sega, and Epic are removing a lot of their games from the Steam store either because they want to replace them with a trash collection, remaster, or because they are anti costumer. Luckily I bought everything I wanted so far.
What a lot of GOGers tend to hate about Steam is the drm (especially when it is disguised as ownership like Steam does it) and also the fact they are the main driver for deep platform-specific integration into games that is hard to decouple (some devs would do it deliberately anyways, but many do it accidentally) and makes games harder to obtain drm-free.
But we tend to hate other drmed stores too. In fact, the only thing that I like about them is that they cut into Steam's margins a little although not enough to prevent it from being it a quasi-monopoly (still, it could be worse).
But otherwise, while some GOGers are very exclusive to GOG, I've shopped around among those stores that mainly/only offer drm-free games. In the past, I've bought games from DotEmu, Humble Store and recently bought some VNs from Jast and am strongly considering getting Celeste from itch.io (I get a feeling the game will never come here). However, the DotEmu store is gone, Humble store mainly sells Steam keys now and the games I bought there are no longer available to download. While those games are backed up, they are not getting updated anymore which has some value and those lessons taught me that all things considered, it is best to buy your drm-free games from a store that is more likely to be around in the future (and in the drm-free arena, that is GOG right now).