I'm really glad to see this post. I think you hit most of the major points, and made most of the strong arguments in favor. Might I recommend you add a section rebutting arguments against? I have advocated for this to many of my friends, and I've heard:

• Isn't this a lot like slavery? Or some kind of fractional slavery?
• How can young children enter into a contract like this that might bind them for the rest of their lives without getting fucked over?
• What prevents parents from maliciously selling all of their child's future income for short-term gain? (And correspondingly: shady corporations from buying it)
• How should this be enforced? What happens if they default?
• Won't this prevent people from entering <artsy field that questioner deems important>?
• Won't this prevent students from becoming "well-rounded"?
• Won't people take short-term gains and not really think long-term about this?
• I'm not convinced investors will help the people they've invested in... (yes, I know this seems silly, but I've really had to rebut this argument)

I worked backwards from "no one in the educational system is directly incentivized to help children grow up into happy, productive adults" and derived that financing exactly like this was the ideal path to building a system that actually works. I'm really glad to see other people thinking these thoughts, and I would love to figure out how to make this a reality.