So I'd be wary of assuming that online chatter represents a "huge" anything.
The definition of a "huge" audience on TV has also changed a lot over the years - what used to be considered skimpy ratings can now be acceptable.
I'm going on the "where there's smoke, there's fire" philosophy. Every time someone mentions Star Trek
or space opera in general, there's a tsumani of postings complaining these things aren't on TV, here and on various other sites. I don't see that kind of passion for other sub-genres of sci fi.
What that means, who knows. I always suspect online posters of being pirates so their opinion doesn't count anyway.
Also, it's very possible that you give them a space opera, but it's not exactly what they want (too dark, too light, the Klingons' foreheads are too turtley) and they'll reject it.
And I often do see people making the sci fi = space opera only mistake. I sometimes try to correct them but mostly I just shrug and move on.
the decline of interest in space opera happened later in the mass media than it did in prose.
I don't think interest has declined. But audiences have fragmented so it's hard for any given show to build an audience. That means the more expensive genres like space opera are out of luck. There's an audience that wants space opera, but the networks aren't willing to approve the budget level required, with ratings declining across the board for everything.