When it comes to cooking, it's hard to imagine someone getting progressively worse, especially when they are the ones tasting their creations. If it doesn't taste right, they'll naturally adjust and improve. The feedback loop is immediate and personal.
"Better" is very one-dimensional, but cooking isn't. You can get worse at taste but better at health. Better at health and taste but worse at time consumption. Better at price but worse at hygiene. Many things are possible, but without an audience or at least your own critical acclaim, you cannot prioritize.
And, of course, "there is no accounting for taste" meaning that some of the aforementioned dimensions are very subjective.
It's easy to imagine people getting worse over time. I see it regularly.
People don't do blind taste tests when they cook. They cook something and compare it to their previous memory. Then they slowly diverge and enter some crazy part of the space where no one should be cooking an ingredient that way. There's no notion of something tasting right on its own. You need to compare it against a better version of that dish.
It's easy for two people to cook exactly the same dish, following the same recipe, both of them to think it came out great, and then when they taste each other's to discover that one is dramatically better.
