We already have specialized conditionals in CSS, such as @supports, minmax, media queries, etc.

if() is just a general purpose conditional.

Using if() is going to reduce complexity for a whole range of use cases. Right now, developers are using custom property hacks to simulate true conditionals [1].

[1]: "The --var: ; hack to toggle multiple values with one custom property"—https://lea.verou.me/blog/2020/10/the-var-space-hack-to-togg...