there is a solution very easy. to avoid art to be stolen in that way, the author should create his own NFT
and not wait other people to stole a work.
This is not a solution at all, it doesn't prevent thieves from making an NFT. They can do it and try to impersonate the original artist, they can pretend to be a different artist and pretend that it was their original work. They can even flip some pixels to fool the reverse image search algorithms. They can scan a physical artwork of some obscure artist and pretend that it's the original work. There's lots of ways to create fake digital art and get away with it if the buyer doesn't put enough effort into verifying it.
Look, I'm not even going to defend NFTs because I totally get the criticisms, but if we're going to use this argument to conclude that NFTs are bad just because some art are getting stolen while ignoring the fact that some digital artists are making money in a difficult-to-gain-recognition industry, then we're just going back to the foolish "bitcoin is only being used by criminals" argument. Let's not use the arguments we hate.
"Bitcoin is used by criminals" is a bad argument because a relatively small share of transactions belongs to criminals, and Bitcoin is not exactly a big crime enabler, because there are better tools, like privacycoins or cash. Though Bitcoin certaintly did contribute to the rise of ransomware, and it's still the most popular payment method for it. But overall Bitcoin is a net positive because its other properties offset the negatives.
I will argue that NFT is a net negative, because it's so full of scammers who pump their NFTs with fake trades and lure suckers into buying something that is insanely overpriced, it creates other negative situations like the one that I posted, and it has very little positive effects, because the technology itself solves absolutely nothing from technical point of view.
Small artists actually tend to lose money, as minting an NFT is very costly because of ETH gas fees, and they have low chance of making profit. NFT doesn't change the fact that few people are ready to pay for art that they can consume for free - buying prints, vinyl records, etc. Those who want to do it already do it just fine with traditional payment channels.
People who made huge amounts of money on bitcoin are the ones that were also already rich. Doesn't make it a bad thing.
Bitcoin gains are a multiplier on the money that you have invested. So two people who invested different amounts in the same time have gotten the same ROI, which is fair. And they had pretty much the same opportunity for investment, as there is no barriers to enter Bitcoin market.
But with NFT small artists get nothing while celebrities instantly make millions. I'm not saying that inequality is evil and we should fight it, but this is a counter to the popular argument that NFT helps the artists. It does not, it generally only benefits those who don't need the help.