But there are also plenty of use cases where recalculation will be valuable in the client. CSS variables cascade so a preprocessor isn't going to be able to know ahead of time what any given variable value is.
Sure, the new syntax allows doing some nifty stuff with the cascade. In practice, however, I foresee most usage being simple one-time transformations of design tokens. I suppose it is more of a theme architecturing issue.
It’s the same with Custom Properties. There are plenty of situations where they are useful at runtime, but a lot of their use is just a single definition on :root, and people really would be better served by the likes of Sass variables, because they foil all kinds of optimisations. You end up with things like color-mix(in srgb, var(--some-unreasonably-long-name), transparent) where it could have just been #1234567f. Quite apart from the runtime and memory costs, bundle size (even gzipped size) can frequently be reduced drastically by flattening eligible variables.