I’m not sure many apps will move to this model. Not because it’s a bad model, but because it’s just not worth it. Current apps would have to be redistributed which means they’ll lose their ratings and reviews (and possibly their current users); and new apps wouldn’t make enough money if sold for less for all the platforms, or lose customers’ interest if they have to pay multi-device money for an app they’d only use on one device. And, as far as I see, there’s no way to use both models and keep everybody happy.
Also, many free apps like Bear use a subscription method for cross-platform distribution, which I’d bet works better for them than a one-time purchase.
Update: Apparently, according to MacStories, the cross-platform subscription method mentioned above was previously a workaround by the developer. Now, with universal purchase, it doesn’t have to be. So yeah, there is at least one benefit to the new model.
Another update: One very important thing I missed was Catalyst and now that I consider it, universal purchase would work brilliantly for Catalyst apps. Because the app was (mostly) written once, it makes sense to also charge the user once. So yeah, there are at least two benefits to the new model.