And I don't need to. Printed pages are only fixed because they physically must be. You might argue that a designer has the expertise and education to ensure the best appearance and readability. And that is very often the case. These are important facts about web publication that adaptability addresses, but which are often ignored.
Adaptability is a fact of the web.
It is a constraint, a flaw, a bug if you choose to see it tht way, or, I would argue a feature, a strength of the medium. It won't go away, and I believe it shouldn't go away. A second argument against adaptability might be raised on ethical or legal grounds. Or from an ethical point of view, a designer might argue that it is rude, and possibly contravenes moral rights provided for by copyright legislation in many if not most countries. You could counter that ethically, the web is about accessibility, and it is problematic to exclude readers from information on the basis of disabilities.