Users will overwhelmingly use browsers in vanilla config. The question here will be how browser vendors show this option. If - say - a company that gives away a browser for free but makes money from ads designed this, then they'll hide the option deep in some obscure menu, never remind people it exists, and reset it on every update.
So the devil is in the details. The best option I think isn't a secret setting in a browser, but a standardized consent dialog. Basically the sites communicate to the browser a standardized data format for consent. Then the browser shows that query in a popup that looks the same for every site. That means 1) the sites no longer have a chance to do dark patterns 2) it's less confusing for end users since the UX is always the same 3) it allows users to check a "Automatically reject for all sites". The site should not know whether the user has auto-rejected this, or manually rejected it. There should be no option to automatically consent for all sites (Can't have that). So the only ergonomic choice is to set it to auto reject.
Having this "use this choice (reject) for all sites" is the really important part here. Because it means that ALL users of ALL browsers will quickly see this choice, so in short order a huge chunk of users will have made this permanent rejection choice.
Dialog is already standardized in the current GDPR. There is literally an item there which states that Reject consent option should be the same and equally easily accessible as Accept consent option. So basically all dark pattern sites are already criminals. The problem is zero enforcement of the GDPR.
It's standardized in behavior only, it doesn't prescribe which button is where in the dialog etc. But yea for those that actually follow it, it's not a _big_ problem, but it's still an improvement to get an exactly similar dialog. Especially if you can check the [x] reject everything on every site
Since tracking is not legal without informed consent, either browsers will be mandated to default to no tracking, or to display a choice on first use. Silently defaulting to tracking certainly won’t be an option, given the whole GDPR and e-Privacy framework.
So the devil is in the details. The best option I think isn't a secret setting in a browser, but a standardized consent dialog. Basically the sites communicate to the browser a standardized data format for consent. Then the browser shows that query in a popup that looks the same for every site. That means 1) the sites no longer have a chance to do dark patterns 2) it's less confusing for end users since the UX is always the same 3) it allows users to check a "Automatically reject for all sites". The site should not know whether the user has auto-rejected this, or manually rejected it. There should be no option to automatically consent for all sites (Can't have that). So the only ergonomic choice is to set it to auto reject.
Having this "use this choice (reject) for all sites" is the really important part here. Because it means that ALL users of ALL browsers will quickly see this choice, so in short order a huge chunk of users will have made this permanent rejection choice.