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.
It 1000% falls under branding. If you don't make errors consistent across browsers / platforms your support staff will politely but firmly burn your house down. There's no such thing as your responsibility ending and throwing your hands up when you're a company who has to do end-to-end support of your product.
Having as little native anything means you go from
n sets of documentation to 1.
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.