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.

Been a shit show in various forms over the years, it's just that the modern iteration is hitting the pockets of the audience now and is blowing up the debate once more.

Used to play in a few bands and the only difference from starting out to my later ones was the closure of local venues making booking shows harder.