None, because you actually have constructive criticism. Most of us defending DC are defending him from the troll-y comments and non-constructive criticism. We've never said it was perfect but we also don't think he's being malicious like a lot of people insinuate. Comments such as, he's holding the update til the first to get his next paycheck, or that they somehow have inside knowledge of his financial status, how much money he actually makes from this, etc..
They didn't do anything constructive and only served to stir the pot. Your sort of criticism, I'm happy to see and it's all valid. SS would probably be better in another engine. From what I understand, DC is currently using a highly modified version of Ren'py. Highly modified can directly translate to highly unstable if the coding isn't spot on, which it isn't. It's one reason I'm not surprised this update is taking so long. He's reworking the base code of a highly modified engine to change the dexterity system, for example.
Whether that's Unity or another engine, I couldn't say. I don't think Unity is very expensive. IIRC, a license isn't required, as long as you're not making more than 100k per year from the game. DC makes 100k every two months, so he'd need a license and that's for each member of the development team. A pro license is 125 USD a month. If he has four others who use the engine, that's 625 USD per month. there's also something called Unity Teams that is cheaper, but I'm not exactly sure how that works.
But, as you said, that will require them to port all that information into a new engine which may or may not be worth it at this point and time.