I think you're exaggerating here. I recognise the "branding is more important than usability" approach, but the OP specifically said "at some point". Have you ever worked for a company that forced a design decision knowing it would prevent anyone from using the product? I suspect not; there will always be some point.

The issue here is, at what point does usability take precedence. Does input validation fall under "branding" or "function"? IMO, an error modal is nothing to do with company branding, etc. — it's not even part of your document, it's the browser's responsibility. The browser can decide to do something completely different from showing a modal as far as its concerned, so you shouldn't make any assumptions. Your responsibility ends at declaring what valid data is.

I think the disconnect here is in the interpretation of “more important.” Usability is more important than branding from the point of view of the victims of this kind of over-designed software, the end users. Because they are already being inflicted with a lot of branding, please at least give them conventional error messages.