Look Mr. i have toilet paper as an Avatar, it is debatable that Ridley could end as boss, but King Dedede?
Besides let us not forget that Ridley has been on all the Smash games in one way or another, and that he has been one of the most whanted characters since Melee.
Also, you're just reading the tittle of the song, on the update it clearly says:
This song appears in a variety of games in the Metroid series.
It’s the theme song for Samus’s arch-nemesis, Ridley.
In the original game, most of the places where you heard this song were the sites of fierce battles.
And that's it, besides the Music Argument is incredibly stupid same with the "too big" argument, so far i haven't seen a good argument against Ridley, well i saw one once that sayd it would be too dificult to animate properly, but Charizard shot down that Argument.
Besides, who is more Important than Ridley in the series? besides Samus? no one. It has always been my fear that Dark Samus could get over Ridley, but i doubt it... Ridley had the same votes as Ike/Diddy, it's a fan favorite and has a lot of potential, making him JUST a boss would be unfair.
Also, Bowser and Ganondorf will probably be also bossess, does that means they whont be playable? i doubt it.