First being Reddit wouldn't have suddenly needed to stop supporting 3rd party apps for free - apps that only cost Reddit money and generate no income. If the API made Reddit money, instead of costing them money, there would be no need for a sudden change. The "suddenness" of this change is due, in no small part, to the cost of supporting basically free-loading 3rd party apps that consume resources but offer nothing in exchange.

Secondly, Apollo would have formed a business model that supported paying for API access, be it monthly subscriptions, freemium, etc. The current model of pay-once-use-forever is simply unsustainable (on an obvious level) - and the $12.99 annual subscription equally so. The Apollo model, as it was, required free API access.

Even if Apollo had been paying for API access all along, and Reddit decided to suddenly hike the prices - Apollo would have been in a better position to raise their own prices accordingly, and would have had a userbase more accepting of paying.

Paying for API access also compels the business to be more efficient in their calls. As it was, there was little-to-no incentive to operate large content caching on your own servers/services, etc. I have seen, but do not know their credibility, that Apollo was not very efficient in it's API calls and essentially hammered the API far more than was necessary. If you're not paying for it, why would you bother designing a more efficient system?

The API fees were inevitable. More are coming - be sure of that, as corporations tighten their figurative belts and look for ways to stop bleeding money.