Sorry to hear you're moving away from Magento. I'm one of the product leaders working on Magento 2; in particular the extensibility platform and want to address your concerns for our next generation product. We've made some sizable changes that I hope address many of the issues that you've raised on the developer experience ( along with others in the Magento community ).
- Rewriting Classes: we've removed the Mage god class and introduced services ( PHP class layer slipped between the m-v-S-c in the typical MVC layer ) to provide a stable interface. Framework also supports exposing service methods as APIs along with a before/after/around concept for service rewriting if needed.
- Front-end UX framework changes: we're now using JQuery, all XML is validate-able ( so your IDE will detect typos ).
- XML; yup still a lot - but now all XML in Mage 2 is validate-able and should be consistently used
- Performance; we haven't worked on yet as we're focused on architectural changes ( Mage 2 code base is in heavy development ).
For paid extension developers and even for us ( i.e. EE licensing ) - what would be the ideal mechanism for paid / licensed extensions?
Have you already and/or would you be willing to take a look at Magento 2 [ https://github.com/magento/magento2 ] and providing your feedback? I'd appreciate hearing if we're addressing your concerns and/or how we can improve the product.