I also don't think things would have gotten this bad if the original dev team could have picked up more of the slack, but I don't have experience in dysfunctional workplaces. Still, the "We had one guy with all the domain experience, then we fired him and all our other devs magically became amazing" thing doesn't sit right with me.
Yeah, because that framing is imo wrong too.
1.) In all likelihood, they Rick was not that much genius, although he might have some more more knowledge initially and probably was not bad programmer either. There is nothing to show he was genius.
Here I will make the guess: Rick belittled other people and criticized everything they did and simultaneously had more initial knowledge. That made everyone assume Rick is genius and others are low skilled. The politics follow from there. Note that other people did not slacked at work nor took three hours long lunches (I assume). Them working overtime would fix nothing.
Local praised genius belittling other employees has predictable consequences - it is that others look significantly less skilled then they are to management (yeah management is to blame too) and those people know it. It also means those people will learn to not have ideas and not have initiative, because they get insulted for those and because those invariably turn against them. As I told, it is very predictable.
2.) The other people did not turned from people who are learned to be passive to people who suddenly got agency and autonomy. Nothing in their programming skills changed, only people management of leadership of company changed.
And note how them not being arrogant still means they get comparably very little credit for technical talent or skill. (I see that as pattern in it.)