You're right in that exposure is the key to success. If you write the world's best song and have no way of getting it to the right people (I.e. people/fans of the sound/genre your in) then it'll die a death next to an average/passable song that people get to hear.

The second element which you overlook is timing. Catching the zeitgeist of the moment is the difference between riding a wave and getting to the beach when the tide is out. If you have a great song but the mainstream audience is tired of that sound/genre then chances are they won't give it the time of day/or lack enthusiasm or will get passed up in favour of a new song with a new sound. Nirvana pretty much did that to glam metal.