I've written my fair share of Duncan/Methos, but to be honest, I'd never be able to put Methos with one person and have him forsake all others. He's got so much history that I like exploring and only a very small fraction of his life has Duncan in it that it's inevitable for me that I'll stick him with a variety of others.
When I started reading fic in the fandom DM/M was the easiest to find, so it was easy to think it was the pairing. I don't think it'll stop being the dominant pairing, but I do think it's not so impossible to find other stuff now.