I believe this is oversimplistic.
I.e. Take 2 investors.
Today, Investor A invests 100% of their net worth in Tesla buying straight shares.
Investor B (following my advice) invests 105% of their net worth in Tesla.
I will bet you that investor B does better than investor A, 5 years from now.
It’s not simply a question of using or not using margin. It’s a question of degree.
I don't agree with you. This kind of thinking gets people into trouble. I bet a lot of people on this forum already got into trouble, it's unfortunate. Investment is a very difficult endeavor, it's not a gamble based on simple math or confidence. But we do know there are sound investment rules, people should study and follow those rules.
Borrowed money has cost. Margin probably costs 5~7%. You are making money for the brokerage. It's a risk free gain for them, but there is no free lunch in the market, so who took the risk? Basically you took the risk, and will take reduced gain, or take a loss on top of paying margin interest. I think most people should treat margin like drugs, never touch it. A little bit margin will turn into a habit then later a lot of margin. I mentioned this a long time ago, the only time you might consider using a bit margin is after the market has been in a recession for 2 years, you start to see the market recovery. Even in that case only very experienced traders can do with strict cut loss rules.
Second, what works on math may not work in the real world. You think you get everything checked out, this is guaranteed to be a 10 fold, then something happened, turns out this is not as great as expected. William O'neil said, in the end you will find ten baggers are very rare, one out of 10 of your picks may turn out to be a 10 bagger. I will add, usually the one you are most sure may end up disappoints you.
When you talk about someone investing 100% or 105% of net worth into one stock, both ideas are crazy. I know you are just using it as an example. If you have a large sum to invest, you need to have a plan, follow sound investment rules, don't gamble. Having the entire net worth in one stock may end up losing his entire net worth, or watch all the other stocks he liked to triple, only the one he actually invested goes nowhere.
If someone can not find 5 great stocks good for long term investment, I doubt he is that great to pinpoint an exact 10 bagger. If he can find 5 great stocks, why not invest in all of them? The benefit of diversification is huge.
However, TSLA might rally, then those with high leverage and concentration could gain a lot. So everything I said is purely opinion, not advice.