IMHO rendering engines can be ignored for restricted use cases or if it's fine to work 98% of the time. What we're expecting from a mainstream browser is a way higher bar, so having no control on the engine is a no go. Tomorrow Firefox having to wait for Google to implement a new sandboxing approach, or not able to override deeper DRM or tracking integration would be a pretty bad situation.
As I understand it that's exactly why Apple took webkit and ran with it.
> Cookie Banners?
People really viscerally hate those, do they. That anger should be pointed to the site pushing them IMHO, but aside from that, dismissing the banner is in itself a legal choice (whatever the default was) that isn't only bound to cookies despite the name. Whatever happens on the backend or service can also be bound to that choice.
I look at it the same way we have newsletter checkboxes. They're a PITA but I wouldn't trust an automated system to make the right choice on every single form, and not sign me to some super weird stuff just because it thought the checkbox was a newsletter optout (imagine a site pushing a "bill me every month for the extra feature" clearly explained option, but with an html input id close to "opt_out_of_free_plan" and it's automatically checked by your browser)