That's a very reductive and limited way to look at art. You're right in stating it as decoration but I would not conflate the two. Different things, both valuable in its own right.

A rose is beautiful, a painting of a rose less so, its two dimensional and lacks many aspects of the true rose. But it has more value because there are millions of roses, while the painting captures a unique experience of viewing the rose by the painter and transmits it to the other person viewing the painting.

Unless there are ghosts in the shell, MidJourney gives an aproximation of how a painting of a rose looks like. Its like an aggrregate function that averages a million artists. Its a weird concept.