They milked it in phases. I'm honestly surprised they didn't actually start the game out at $0.99 and then drop it to free.
Each phase, they're targeting a different demo. In the first phase, they went after the early adopters. They could have easily charged 99c and still had IAPs. But anyways, with keys and stars, they wanted to get everyone playing, sharing and reviewing it.
Because it would be mostly core players jumping on early, the difficulty balance for casuals could be corrected with money.
Then once that audience tapered off, they patched out the gates, and made it into a difficult linear adventure that straight up requires cheating, breaking all the balance. They don't give a shit about preserving the previous players. Now they just want to frustrate people who have a few worlds left, and make them give up their cash.
IE: "Dammit, I'm only a few worlds away, I'll spend the $20 on those gems and be done with this."
And now they've seemingly passed that phase and are now just in full "Fuck you, pay me" mode.