The best way to not improve is to avoid feedback or operate in a vacuum. Let's call it operating within games vs. echo chambers. I use game as shorthand for the idea that there are well-defined rules and criteria upon which feedback is given. That feedback could be qualitative in the form of a conversation, or it could be a score in a game. Feedback is an audience clapping, a boss telling you that you did well, more money in the bank. An echo chamber is you buzzing around in your own head without some form of feedback.
From my own experience it's obvious that my progress comes when interacting with other people or along an axis of progress such as in a game. For example when I started building my first product I was not familiar enough with the rules of the game to know when things were vs. were not working. There was no feedback loop - I now chalk that up to raw experience. It was less a game but rather an echo-chamber of me bouncing ideas off myself or very close associates in the very early days. There weren't enough (any!) customers in those days to validate we were on the right track. Grit and perseverance were the only reasons success was found in the end. But the cost was too high in time and sweat. So the lesson to draw is that if you're operating in an echo chamber see if you can pull in a feedback loop to ensure continuous progress.