Hi Tim
Yes Brian Wilson never reached the song writing heights of the Beatles as his artistic growth came to an abrupt end around 1967 and by 1968/69 he was admitted to a psychiatric hospital for serious substance abuse issues and resulting mental breakdown.  Interestingly he was 26-27 (born in June 1942) during this period and came dangerously close to being another Club 27 casualty to substance abuse which seems to have snuffed out so many great musical stars before they could shine their brightest.  I read up a little on the Club 27 phenomenon, and the most common theory is that around 27 the body can’t take the ongoing substance abuse that lead up to it from late teens to early 20s. All the abuse finally catches up by about 27. If you look at it, just before 1968, Brian finished his Pet Sounds Album, released Good Vibrations (his two greatest achievements), and started on his 12th album “Smile” which was supposed to be his response to Sargent Peppers, and anticipated by many musical critics to be the makings of the next monumental album to eclipse the best Beatles Albums to that point, but Brian was not able to finish Smile because of the substance abuse and the Band released a half-baked unfinished Smiley Smile album that basically flopped in the charts. It was a great tragedy, so much unfulfilled potential for Brian Wilson, the world would never witness.
On a positive note, I saw their 2012 50th anniversary concert tour (on TV), and it was inspiring to say the least to see the entire Beach Boys band (Brian Wilson, Al Jardine, Mike Love, and Bruce Johnson) touring the world and putting on such a great show. To see these men all old enough to be my father still rocking and rolling was very inspiring indeed, and blew a little breath of fresh air into my life at the time.  Apparently Brian is starting another concert tour to commence Nov 30th this year. How amazing! And didn’t Paul McCartney release a new album this year?
Music is indeed a never ending life time journey as these two great legends prove.  So many decades later the Beatles and Beach Boys still touch and move people’s lives with their music. What greater testament to greatness is there? And I am thankful there are still a few bands like the MonaLisa Twins carrying on that tradition of great music making that started in the 60’s. So yes, I see some Brian Wilson, as well as a lot of Beatles greatness in the MLT.