Okay, people who know more about finances than I do... tell me what's wrong with this concept.
Imagine there is a country called Investopia. And every citizen of Investopia, when they are born - or when they immigrate and gain full citizenship - is given a special bank account with a million Investopian dollars in it.
(The actual amount is pegged to inflation, but for simplicity's sake, we'll call it a million bucks.)
Now, this money does belong to that citizen, but what they can do with it (or what their parents or guardians can do with it, if they're underage) is limited. The money in that bank account can be invested however the citizen likes, but only in Investopian funds that are managed by Investopian financial institutions that are, in turn, insured by the Investopian government. You can select different types of investments from different private institutions and banks, but they all have to be strictly within the country, and what you invest in is required to be diversified - no more than 50% in any one place.
(Most people, presumably, don't manage their own portfolio - they just wave their hands and some computer program shuffles their money into index funds, GICs, bonds, etc for them.)
At any time, the citizen in question can take money out of their bank account - of course they can, it's their money! - but they can never reduce the balance of the fund to less than a million dollars. You can leave the money in your account and allow compound interest to do its thing, or you can skim the interest every month and hey - if you're thrifty and your investments do well, that's enough to live off of!
(Of course, it's not impossible for a diversified portfolio of index funds to lose money, so your balance can drop below a million that way... it just means you won't be able to withdraw anything until it builds itself back up.)
Once the citizen dies, the government simply takes its initial million dollars - the money that was invested in that citizen - back. Whatever's still in the account, of course, is property of the deceased's estate. So, presuming a zero population growth, the government never loses money on this program - they put in a million when you're born, they get a million back when you die.
And, of course, the citizen is still free to spend money elsewhere normally - normal bank accounts and riskier markets exist outside of this structure.
Functionally, Investopia has a Basic Income, but it's not tax-payer funded - the government just makes an one-time interest-free loan in each of its people, and collects the seed amount back post-mortem. While those people are alive, they're investing that money in Investopian companies and resources, providing plentiful capital for new business opportunities.
Now, I admit, I know nothing about finances. I don't invest (I don't have a million dollars to invest with), and I haven't taken any sort of classes in the subject. So I'm asking you guys - what is wrong with this model? When and where and how and why will it inevitably break down?