Your arguments seems reasonable and I heard it before, but I am not really convinced it strikes to the heart of the matter. Accessibility does include more than the HTML Spec, but adherence to the spec already provides a huge chunk of accessibility for free. And I don't feel every website needs to be 100% accessible, but people complaining about lacking accessibility often do so because developers disregard good practices and common sense. Yes, bad contrast is bad, but most disabled users know ways to fix this minor issue[0], but needing js to dynamically load basic text and image content that a screen reader than struggles with is just inexcusable and gets to the hearth of the issue, that ignoring accessibility also often means ignoring different user Plattforms and usage. A finely crafted website is not only readable by a modern browser and a screen reader, it will also very probably mostly work on older browsers, minor browsers/operating systems and so on. So every developer should have accessibility in mind when creating websites.

Of course, being competitive, a shoddy developer who just bangs two wordpress themes together will always be cheaper than good craftsmanship, but in my experience, the problem is more often targeting the wrong clients anyway.

[0] And, as sad is this is, the actual design is often not the developers responsibility, unless of course, when it is, in which case he is also to blame. ;-)