CSS variables don't impact CSS' simplicity. If you need CSS to be simple, its still gonna have that, but when you get around to needing:

css variables, transforms, gradients, border-radius, multiple background images, etc.

... you'll be glad to know that those features are available as well and that you don't have to add complexity to whatever you're building to hack those in. I also don't see how CSS variables, or other recent CSS features, hinder anyone from learning from a site's code.

All these things make it easier to create stuff. My only complaint is that they're not added soon enough.