[JJ_SO]

Back in the film days, many people were willing to pay for a booster or even viewfinder ...  
Today you can at least buy vertical grips.
External flash units are nothing else but customizations. External mics ...
You can buy external viewfinders for some of the EOS M cameras.

It's not as if you can't customize your camera these days.
In all seriousness - why shouldn't it be possible to make the viewfinder modular?

Of course, I don't know the market size.
However, I think that we all have our no-goes when it comes to deciding on a camera. And why shouldn't it be in the interest of camera makers to provide solutions for "easy" aspects.

Modular accessories? Here I'm with you. I'd also like to see interchangeable sensors, EVF, LCD. If that (adding stuff to a camera) was you idea in the beginning, I obviously misunderstood your idea completely.

But as you're also a big fan of "weather sealing", you might ask yourself if you're willing to take the weight of added sealings as well as the necessary maintenance?