They're not "essential," because plenty of strategy games do without them. EU2 and CK have no industrial development model. It's outside of the period. CK doesn't even have trade, and while I would like to see trade implemented in a future installment of the series, I can't say it's not a strategy game because it lacks a real economic dimension.
Fond of events, aren't you?

There's a lot you can do with them, but events are not a mini-game (how do you "win" at events?) and often they are an inelegant brute force solution. I would rather see these things integrated organically into the game.
Why do you say that? Do you think that trade was some wondrous invention that people did not discover until after the Middle Ages?