I think you might be underselling how important things like the steam workshop and steam’s multiplayer support are.

Games like Starbound or Don’t Starve benefit a lot from the workshop.

While insert any party game gains a lot out of steam’s multiplayer support and friend list.

Also, while I don’t use Linux myself, Steam is one of the main reasons why Linux Gaming is a thing.

Imagine caring as a value, for example, my care™ about this topic is 2, whereas yours is 5.

In this interpretation, “I don’t care” implies that the care™ value of the speaker is 0, by the same logic, saying “I care” implies that their care™ value is greater than 0.

With that in mind, “I could care less” implies there is a care™ value lower than the one they currently hold. Meanwhile, “I couldn’t care less” implies the opposite, there is no care™ value lower, which is only true for 0 (AKA “I don’t care”).