For what it's worth, the node icons are using Font Awesome too, as are the post editor icons and surely several other components. Maybe a disable option is viable for the header/navigation bar, but I think the icons will be too integral for something like a complete global disable.
Yeah, I'm not really keen on the inconification of every button some themes have these days, especially with all the private control buttons below posts.
The only advantage of having both on larger screens is you learn the association of the icon to the text, so when on a small responsive screen you can get away with using just icons.
That's still not fool proof though and I still kind of prefer just text in most cases, it's generally cleaner looking and removes any ambiguity, as you may still wonder what an icon it's own may mean at low res.
The places you can get away with it are on user account controls, so inbox and alerts, but there I feel you're better off removing the text completely.
I think what the XF team have done currently is probably the most flexible at the moment though, having both out of the box, so only a small edit is required to customise it.
Personally I'd end up removing all the icons from the navigation buttons on the left, leaving clean text like your mockup Andy.
Then remove all the text from the account controls, leaving a much tighter, icon only alerts section on the right.
PS. Generally I really like the look of the new style
I'm going to say that it's unlikely that we'd have a global switch. In XF1, there aren't global switch for all icons (forum icons, thread status icons, the search icon on small screens, etc) and a global switch here would be equivalent. Additionally, even things like the sharing icons aren't images, so a global switch would invalidate that.
A potentially implementable suggestion would involve switches for specific types of icons you want to remove.
Studies have generally shown the icons are better for power users while text is better for new users. When a user knows what an icon means, it is quicker to differentiate them (compared to lists of text) and select what they want. However, as noted, it might not be clear what the icon means to a new user. Mixed approaches (icon + text) get the benefit of both, primarily at the expense of space.