You seem to be tying marketing into the definition of a product, as if a product cannot be great unless it's well-known. This makes no sense. A product can be great and still be obscure relative to competitors, even inferior ones. cf. Windows vs. Linux, IE vs. Firefox during the browser wars, etc.

It's the same old story: OS/2 was superior to DOS, Netscape was better than IE, Dreamcast was amazing, Amiga was ahead of its time, etc.

It's not just software. Shakespeare wasn't a popular playwright in his time, but now he's considered an icon. Were his plays therefore not great because, at the time they were written, no one wanted them?

Many good things are not popular or well-known. They are no less good.