That being the context, I've seen tons of solo-founder businesses do very well and come up to that cruising altitude with hard work and dedication. And that's OK.
Also, the OP's point on young-inexperienced solo founders vs. older experienced founders resonates with me. All I've done is solo and self-funded. I've made a ton of mistakes. I've made a ton of money and lost a lot of it. If I were in your shoes I would not, in a million years, hand the keys of a startup to a young just-out-of-college kid. Business can be brutal. It is very, very easy to become your own worst enemy. If you have a small team motivated to succeed because they each own a piece of the thing they are building they'll probably help each other stay on point and get it done. Of course, they also need extensive external coaching, support, guidance, etc.
How many of your startups would have succeeded if the infrastructure you provide (beyond just the initial small funding amount) did not exist? My prediction is that most of these teams of young founders would crumble under the pressure of doing a business without the team that you put into place to make it happen.
Example: I have a friend who started a very successful courier company. He now employs north of a hundred drivers. No formal education. High-school drop out. Makes loads of money. For nearly a whole year he lived, ate and slept in his car as he service this one mid-size client who gave him a shot making deliveries. He literally slept in his car in the parking lot outside of their offices and workers would come wake him up when a delivery had to be made. No support system, no coaching, no, well, nothing. I don't know many people who would be willing to endure such torture.
My very first startup. Self-funded it with $250K I made after a heavy couple of years of consulting work. My own money on the table. Had my first kid. My wife didn't work. I then spent a year and a half in the garage working 18 hour days --seven days a week-- writing code, designing electronics and everything else needed to get my first product done. It was not fun, but it was a lot of fun. Weird statement, I know. Again, I don't know many people willing to do something like that.
I am not saying that all solo founders engage in heroic acts of self-sacrifice to bring their ventures to flight altitude. But I do think that it isn't quite fair to characterize the solo-founder equation as a formula for failure when the vantage point from which this conclusion is reached is one where there's a huge enterprise and ecosystem behind the startups to provide funding, coaching, guidance and all that might be required in order to succeed. Take all of that away and maybe we can compare apples to apples.