Hard sci-fi can definitely be popular, The Martian and The Three-Body Problem are two examples I can immediately think of. I think the Arrival and Contact movies would also count (not sure if the books were considered popular before their film adaptations came out). There is usually a way to avoid most of the "opaque wall of gibberish" so that there is just enough for a technical-minded reader to tell that the author has put some thought into it and the science makes sense, but still little enough that a non-technical reader can enjoy the story without having to care about the scientific worldbuilding.
I think Lord of the Rings might be a good analogy. LotR is sort of "hard fantasy" in that Tolkien put a ludicrous amount of work into building an internally consistent world, as you can tell by The Silmarillion, but that book is not enjoyable to read (in my opinion). Part of the reason LotR is good is that he took out enough of the walls of text to make it fun to read. A good hard sci-fi author might have a Silmarillion-level of knowledge about their own book's setting, but be able to leave almost all of that out of the final product.
Three Body Problem is not even remotely close to hard sci-fi. The whole notion of "sophons" is ridiculous, for example, and the more the author tries to explain how it works, the more nonsense it ends up being.
If you want another example of actual hard sci-fi, I would suggest Lem's Tales of Pirx the Pilot. To give an example, I remember it being pretty much the first sci fi book that not only pointed out that laser beams are invisible in vacuum, but actually made it a major plot point of one of the stories.
I think Lord of the Rings might be a good analogy. LotR is sort of "hard fantasy" in that Tolkien put a ludicrous amount of work into building an internally consistent world, as you can tell by The Silmarillion, but that book is not enjoyable to read (in my opinion). Part of the reason LotR is good is that he took out enough of the walls of text to make it fun to read. A good hard sci-fi author might have a Silmarillion-level of knowledge about their own book's setting, but be able to leave almost all of that out of the final product.