I personally think the initial reaction was right. If you have a technology in place that makes it difficult to onboard new developers and adds 50%+ to all the development time (we call this the "typescript tax" on the project I'm on), then the time it's supposed to save you on the occasional thing it does catch is completely blown out by all the time you spend tracking down a hidden type so you can put it in a variable that's assigned in a different scope to where you're using it or figuring out how to somehow assign that variable in scope so you don't have to know the type.

And it doesn't protect you from poor design or fuzzy thinking (and all the mental clutter added by TS make those things way more likely).

And honestly a technology isn't great if only the most determined developers can learn it. All that time you spent learning why string isn't the right type but extends string is could far better be spent in learning how to actually be a good developer if you're not, or implementing the business requirements if you are. And it's certainly not great if it's running otherwise good developers off your team.

I also agree with the author. React is built on the core concept of immutability, and MobX takes that and tosses it out the window. If you want mutability, stick with AngularJS. Don't go to a technology that deliberately excludes it and add it.