I still remember David Gerrold talking in The World of Star Trek about the difference between format and formula. Format is the general subject matter, structure, and approach of a series, the basic "flight plan," as he put it, as a starting point for storytelling. Formula is when the execution of the format becomes too repetitive, lazy, or unimaginative, just repeating the same pattern instead of exploring and experimenting with the potential for variety within the format.
It is absolutely wrong and downright condescending to say that all space opera is limited to a set formula. That's what happens when it's done wrong. It is emphatically not the whole point. A genre or a format is just a starting point, a foundation to build on, not a straitjacket.