You've expressed here for what conservative economists call the "Juncker fallacy." I won't get into the derivation of the phrase, but the idea is this: inflated asset prices are not actually costly unless they divert real resources. The 6 million dollars doesn't just disappear. It goes into the artist's account, and then he spends it, and it circulates. Or he doesn't spend it, but invests it, and then it circulates.
Since the artwork is almost free to create, it means that it's considerably less wasteful than other art forms. Some movie studio dumped $200M into a Joker movie that I've seen commonly described as unwatchable. That $200M is, for the most part, real -- for instance, the video editors or special effects artists who worked on that film could have been doing something else productive, instead of slaving away on a vanity project that should have been killed early in the process.
I don't disagree with your characterization of this banana art, or some of the other minimalist trends (which are largely in the past). I don't think it's question of effort, though. Who cares if Leonardo spent a year on the Last Supper or gulped down some espressos and did it in an all-nighter? The point is the feeling or effect it has on the viewer. I'm with you, in that chromatically ordered monochrome canvases bore me -- I'm a Jackson Pollock guy, not an Ad Reinhardt guy. But is it worth getting worked up about it?
If the guy made his money in crypto, then even better. Worthless data table entries traded for ephemeral banana art. Seems fitting to me.