Well written and sobering.
I thought everyone was moving away from Backbone?
I'm sure that many devs are more interested in more modern frameworks like React, Vue, Angular, Ember, etc. but backbone is still being actively developed and I'd argue it's still plenty relevant. Backbone is a much less opinionated framework than those mentioned, and it's pretty flexible and lightweight as well. Honestly, if we weren't working in WordPress I might have chose something else, just as an opportunity to play with some fancy new tech, but backbone is baked in to WordPress which makes it a very sensible choice for our plugin's UI.
We've used it quite a bit recently too for non-wp projects and just read that people are moving away from it, which prompted my comment. I actually didn't know it was baked in. That does make a lot of sense then and it is an awesome framework to work from.
Instead of using this.model.on, you could use this.listenTo(this.model), which allows you to use this.stopListening(this.model) and release all of the view's listeners on the given model, which may help simplify some of that code. Also, some of this item management code can probably be simplified by using Backbone.Collection rather than a simple array, as it gives you some event dispatches when items are added and removed, which you can also use to free up memory, if necessary. Nice article!
Good