Minification would be indeed useless under that set of assumptions, but the real world is much more variable and you need a comfortable margin. For example, mob

Minification would be indeed useless under that set of assumptions, but the real world is much more variable and you need a comfortable margin. For example, mobiles rarely have that large bandwidth sustained all the time. Comprehensively speaking, the minification is only a small step in building a performant website or web application. You have way more things to do, for example choosing a correct image compression format and method would have much more impact in general. But not everyone can be expected to understand them in depth, so we have best practices. Doing the minification therefore qualifies as a good best practice, even though it would be just a single one out of many others.