Just as I'm baffled by people who insist that the verb "discover" may only be used of the first person ever to find something, which would e.g. render it impossible to say that you'd discovered a great new restaurant, I am baffled by people who insist that the verb "matter" may only be used if the attached noun is the only thing that matters.
I refer, of course, to those who take tremendous offense at the slogan "Black Lives Matter," who may well be the same people as those who take tremendous offense at attempts to politely acknowledge that not everyone is Christian by saying "Happy Holidays" when addressing strangers.
It's not necessary to talk about other lives mattering; those are not in dispute. But there is plenty of evidence that, in the U.S. today, in many circumstances black lives are treated, intentionally or not, as if they don't matter. Therefore it is necessary to remind us: Black lives matter.
Whence comes this silly belief that the phrase is preceded by an invisible "Only"? I think I know, but it's not from any previous use of the grammar. I just today remembered that nine years ago I attended a one-off academic fantasy-lit conference called "Fantasy Matters". When I first saw the name, I wasn't sure if it wasn't a plural noun, but no, it's an intransitive verb.
But nobody went around complaining that, if fantasy matters, we were somehow saying that other forms of literature didn't matter. That would have been silly. So stop it.