I was surfing Google for SimplePlanes builds, when I came across Jundroo on game-stats.com, and found a list of their games and the revenue from them.

Given that SP1's revenue over 9 and a half years is around 2.3 million USD, and Juno's is 1.1 million USD over just 2 and a half years (despite a smaller fanbase, I assume), I think SP2 will surpass both games, especially with the features in the development showcases. For now though, we wait for news of its release.