Day One's change to subscription pricing model is taking some flak but I think it's a good move for them and ensures that it will be around for a long time to come.
Subscription pricing isn't new to apps, but it's on the rise and this is largely in part because upgrade prices alone for apps are not sustainable.
A great example of app pricing in my book is the Todoist app. While the Todoist app itself is free, they also have a premium subscription which really adds value what you get from using Todoist. It's subscription models like this that are the way forward. Paying for the software you use on a regular basis. In a lot of cases the pricing is very reasonable and I certainly wouldn't argue over paying between $20 and $50 per year for software that I value and use on a daily basis.
David Sparks rounds up the changing landscape of app pricing nicely.
The traditional model for productivity apps was the upgrade price, where developers released a new version every year or so and everyone paid a reduced fee upgrade price. I know the App Store has made improvements over the last few years but, having zero inside knowledge, I can’t help but feel we will never see upgrade pricing in the App Store. In the meantime expect more quality apps to go to the subscription model and, if they are apps you love (or even like), I’d encourage you to support them through the transition.
— Productivity Apps and Subscription Pricing by David Sparks