I'm a functionalist. A words means what the majority of people using it think it means. If you think space opera means bad sci-fi, then what do you think you have to contribute to the discussion at hand?
It's all relative.
There are a group of people, the "cool kids", that get to label everything....and then the rest of the country just accepts it and uses the label. See for example: History.
So set the Wayback Machine for many years ago: The soap companies make some short drama shows "targeted at housewives" around their commercials for soap products. The shows are bizarre to the extreme, with things like long lost twins, people randomly getting amnesia, people getting trapped in "other" countries, dead characters "somehow suddenly" coming back to life and more.
And someone who did not like such stuff at all, slapped the label "soap opera" on them as an insult.
A couple years later, another someone who likely hated sci-fi (it was very popular to hate sci fi wayback when) slapped the label on the sci fi they really did not like as "space opera".
But that is just them.....the people sitting far away, judging things and slapping labels on things they don't like.
But if your a fan of either or both, then it does not matter what they say: you like it, you are a fan. If you are a fan it's "good enough" and you might even like all the stuff.
It never matters what "they" say.....