In reply to Dovy.

Essentially the customizer in theory is not to blame, but learning a new API, a new way of doing everything, and porting all that code over is going to make every established theme dev cry. :P

That really depends on the quality of the underlying code, and the approach taken. Anyone using an options framework library, or who otherwise uses an array-based options development will be able to port those options to the customizer with a couple hundred lines of code. I was able to do it with about 100 lines of code:

https://github.com/chipbennett/oenology/blob/master/functions/options-customizer.php

If, on the other hand, options are not array-based? Yeah, that's going to be quite a bit more difficult. But the real problem there is using inefficient coding practices that don't scale. And honestly, I don't think there are many developers at all anymore who would fall into that category.

And then there are the Themes that are doing non-Theme things with their Theme options.

As for making established Theme devs cry? I actually enjoyed learning a new API, when I decided to incorporate the Customizer three years ago.