If you want to keep it simple there's that :
According to Steam DB right now
Starbound - 2,361,044 ± 38,248 owners
Terraria - 7,602,158 ± 68,072 owners
15 - 20 k players a day doesn't represent the same for the two games.
I mean Skyrim for instance that came out the same year as Terraria still averages around 32 000 players a day these days but compared to how many it sold according to Steam DB it's a smaller percentage of players that still play it relative to the possible playerbase than those two games.
Something more interesting and maybe more accurate would have been to see how the two games both did at their respective launch, and if they both dipped as fast or not.
And even then with the price difference between the two, with Starbound being in Early access for so long, and with the 5 years difference between the two releases you might have other factors coming-in to blur the lines.
That being said writing this has made me realize how low the percentage of daily players are for games xD
Even a recent game like No Man's Sky only sees around 6% of it's playerbase play today for instance. (yeah I know controversy and all, but still)