I really doubt this will have much effect on any of the competing apps. Brushes was already downloadable for free (with an optional in-app purchase). Many other painting apps of various capability are available for free too.

Perhaps more damage will be done in the vector space, where there's not a lot of money to be made in the first place... but I'd still be surprised if it puts anybody out of business.

At any rate, I don't believe anyone is entitled to any particular business model. More people are better off with open-source software.