Which is why I said "True" artists. There are plenty of hacks. Mike Love hated the fact that Brian Wilson wanted to grow. Love wanted the Beach Boys formula of songs about cars and girls to continue. Wilson wanted to explore on "Pet Sounds". It's regarded as a masterpiece, but wasn't a commercial success at the time. The late 60s was an amazing time for musical growth. Miles Davis, Beatles, and countless others broke new ground.Not uniformly or universally true. There are artists who like to have hits and once they do, they follow that general formula that got them there.
Some artists start with a formula, and then go off on tangents and angles: witness the four split up Beatles.
Other artists, such as Niel Sedaka, would thrive on popularity and look for the next hit, not "something different". And there were artists, such as the majority of Motown stars or the Phil Spector-mentored "wall of sound ones, who followed a producer or mentor's guidance (and there are many who stopped being "guided" and became musical road kill).
Another example: Barry Manilow. He did commercial jingles and did not think of himself as a mass-appeal singer. He liked preparing clever stuff to sell products, and fell into being a hit artist. Now, he thrives and enjoys doing his stage shows because he likes making his audiences have a great experience. I was never a Manilow fan when he had all the hits, but I saw two of his shows at our local venue as part of my membership subscription and realized how much he liked putting on the show for us... from dress to staging to the songs he was "into" making the audience happy.
So to say that musicians are not motivated by anything but art is very oversimplified. Any artist back in the 70's who broke into the Top 10 on WABC or KHJ or WLS was going to be pretty excited and thrilled that so many people were hearing their song.
I agree that not all musicians are motivated by the same things...