25 maybe, but not 15 I would say.
But does it really just matter whether most people have heard anything about someone? Surely it matters just as much how important they are to the field they were in.
Romero was very famous to anyone who likes their movies. Don't have to be a horror fan. Just a movie fan would do it. And there wouldn't be 28 Days Later and the Walking Dead without him, so I dare say an awful lot of TWD fans will most certainly know of him and admire him too.
Been a few deaths on here announced where I have had to look the person up, but they have still deserved their place for what they have achieved and the legacy they have left.