Well it IS possible.
Nintendo needs to have 12 games that sell more than 18.16 million before anybody else gets one. So far they've done it 13 times and nobody else has ever done it.
I think they'll have the top 15 games in under a year or 2, but at some point... SOMEBODY should make a game that sells that much. It just seems unlikely that nobody else could ever do it ever, but I guess nobody else wants to make evergreen titles or something. I have no idea.