I have actually answered this question multiple times over the years here on ET. Most people can't follow a trading system. They will mess it up, even good systems that make money. If the system loses 2 trades in a row, half of the people trading it will quit. You get a drawdown of 10K, even if historical max was 15K, and almost everyone will quit.
What does this have to do with me selling systems, actually everything. I can sell even 500 copies of a ES system and not effect my slippage because only 50-60 people would have taken every signal exactly at the point the system did. I can use this income to fund research and development.
I have a number of people working for me in programming, development of TraderStudio, research in Machine Learning and development of Institutional trading models. Developing, researching and deploying systems for the retail trader is also part of my business. I am able to give the retail trader the benefit of my endeavors in the above at a reasonable price.