I don't really mind ExtJS or Smarty, I'm just sorry we can't update them to the latest versions.
If MODX gets rid of those or builds an entirely new manager with other tools, you may as well call it a brand new CMS cause everything is starting from scratch at that point.
What can be done? I think it's obvious MODX can't survive forever on the existing manager, it just won't. It will slowly hemorrhage users and put off potential plugin makers. ExtJS needs to update or go away so plugin makers have a clear path what to do next.
The way I see it there are only a few paths to take:
1) MODX can fork into another product and Revo gets some long term support for another 4 or 5 years for bug fixes only.
2) Someone has to build an incredibly sophisticated abstraction library to convert everything ExtJS into the new version so that these core libraries can be updated and made use of. Give plugin makers a couple years to convert their stuff to the new versions before this layer is removed entirely. Simultaneously they have to build another abstraction layer to translate the new ExtJS so we don't end up in the same situation again. Building two huge abstraction layers is no small task.
3) Keep everything as-is and watch people slowly leave to faster moving products. Other CMSes are figuring out how to build easier APIs, more real time interaction and development environments (page builders, frontend editing, etc). All the RESTful APIs, headless features, CLI tools and other tooling, custom field management (bespoke content types, matrix fields, repeat fields, etc).
Now, not to be a debbie downer, I think MODX3 is doing great. I like the move the Composer libraries and the UI improvements are welcome. A bump up in PHP version and all that. But it must be answered, what is the long term vision? How is ExtJS going to get updated? Will there be a new Manager? As of now, it's really difficult building custom data, CMPs, working with security model, and some other tasks. All these important things get handed to plugins which themselves are getting complex to use. We can't even get a self-updating tool in the core despite this being a common feature in the industry, or even a backup tool really. The app store is getting really out of date, don't know if older plugins are still safe or not, or maintained at all (which might be a good argument for starting over).
I think the problem is that everybody wants MODX to make all these great leaps in features and updates, but we want it all NOW. We want a huge evolution in a short period of time, and there aren't the resources and manpower and cash to do it. So it's my fear the slow pace and stagnation will just hemorrhage users for more modern options. I never liked comparing MODX to Wordpress, I don't believe they are in the same category of tools, but I do often hear comparison to frameworks like Drupal, but even Drupal made some recent leaps on architecture. What is MODX's next big move to keep pace in the industry? Simply waiting on the open source community to develop these huge leaps is not going to work. These aren't bug fixes and patches, this is a huge core undertaking!
Anyway, keep up the good work, and long live MODX!