The 'growth' framing isn't very convincing to me because growth is something to worry about for a finished game; a game that's still in development just needs to worry about staying funded to completion. Then launch strategy is more important, and then maybe once the game is actually out for a while, growth is something to think about. Starsector is a tiny indie passion project, not a multimillion-dollar revenue machine, so it's fair to assume the goal was never to maximize growth.
I think all the arguments against Steam are still valid; Steam is still very much the PR hub where you put your game to get noticed, even if in real terms there's been a decline for most devs in the number of actual sales/total profit they get from Steam (I'm working on a few things myself, so I've done recent market research on some of this). So not being on Steam is less likely to hurt sales, but being on Steam could affect launch PR in a negative way; I'd love to have it on Steam too, but I can't look at it objectively and say I think it's the right move for the game at this point.
However, Alex, I do also think the arguments against having Starsector on a different platform like GOG or possibly itch.io just got a whole lot weaker after this weekend. Unlike 0.8.1a, I'd say the game as of 0.9.1a is a very robust experience, clearly very near to being complete, and while Steam would be far too much, having a different secondary vendor platform involved really might be a net benefit at this point.