Hi there!
First off, welcome to GluJS :). I know what you mean about activity on the framework, however I haven't had a need for any changes to it in well over a year now. At my last company, we were using GluJS and ExtJS 4.x for the entire application with hundreds of screens, configurations, and custom controls including a form-builder which was a simplified version of sencha architect that non-developers could use (think survey monkey). In all that time, I never had any problems with the framework, so that's why you see such little activity on the code. If you have any feature suggestions, there's always pull requests to get things fixed up... but its been rock solid for my use for years now.
As for ExtJS 5.x support, I don't know what the official word on that is. At this point, I can't think of any reason why it wouldn't work out of the box. From what I've read, all the controls still use the same configurations and have the same life-cycle that they used to. There might be a few tweaks that Mike might have to make to some of the adapters, but honestly I would suggest trying it out and if you have any problems with it... report the bugs or make a patch and do a pull request. I know that everyone out there would be grateful for any contributions you could make to the project.
I hope that helps! Have a great rest of the day, and again... welcome to GluJS!
-Ryan