One of the most beautiful things about music whether a Byrd Mass, a Mozart Symphony, a Beethoven quartet, a Mahler song, Stravinsky's Rite, is the element of mystery these works contain. So, when you ask how important it is for music to be accessible on first hearing, I have to say that the psychology of listening is very complex. I do believe however, that all music of substance should have an immediacy about it. It should convey an aura of significance, which is different from accessibility. If it has immediacy, an impressive presence, access can follow with effort and with great reward.
Instant gratification is only a small part of music's great treasure trove.
If you got everything out of it at first hearing, you wouldn't ever need to hear it again. We are attracted to enigmatic things such as nature, gravity, the cosmos, space travel, God and religions, advanced math, myths, love etc. I believe we find such mysteries in art. Simply open your heart, ears and mind and listen YOUR way. It is the most obvious thing to do.