> hard to quantify
It's hard to quantify perfectly but we already quantify it imperfectly during salary negotiations. Don't make perfect an enemy of good. We could get a better system _today_ overnight if we just took everyone's salary and used it as weight/skill for distributing ownership per unit of time. We could further improve it by renegotiating on equal footing.
I am not against UBI as a safety net system so that everyone has enough to survive. But instituting UBI before restructuring the ownership system would be actively harmful because, again, we need enough straws to break the camel's back so that people take the time and energy to understand the root causes and oppose them. (Because people's reaction is not linearly proportional to inequality - there's nothing (acceptance/indifference) for a long time, opposition forms only when it's sufficiently bad.)
One large underlying cause of inequality is that we have 2 different reward systems:
a) Fixed money per unit of work (usually per unit of time or per item produced).
b) Ownership which gives full control of the owned structure and therefore the ability to capture the full value produced by it. (Minus money to pay workers but money per person does not scale, ownership per person does.)
These map pretty cleanly to the worker vs owner divide. And this distinction is what we need to erase to erase the class divide.
> do I keep the stock when I leave
Yes, that's the point and this is where it would be better than current co-op systems. Every person's economic input into a collaborative effort is the weight used to divide their ownership. So if you stop working there, you keep your part but it keeps getting smaller relative to the rest as more people keep putting in their work or money.
Money (investment) is a valid economic input and should weight towards ownership. How much? We could use median salary of the country, median salary of the company, or my favorite - divide the investor's total net worth by the number of hours he did worked - this would somewhat erase the advantage rich people would have upon transitioning to this system.
> Am I punished by losing stock if I'm fired?
Interesting question - I don't think so, if you used to contribute positively, you should keep the reward, but you might need to be penalized if you caused harm to the company proportionally to the harm.
> How much of the company is owned by former employees?
That would increase over time up to a plateau as they got old and died. (Ownership should not be heritable.)
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Regarding diversifying investment - you can do that by either working for many different companies or by using your money to invest into other companies.
Thinking about this as buying and selling shares is IMHO misleading - it's more like re-weighting a distribution. Adding an economic input reduces everyone else's share slightly but since that input (hopefully) leads to more revenue, they will be better off (if they don't think so, they (as owners) can vote against taking the investment).
I am not an economist and I still feel like I am scratching the surface of how the economy works so maybe there are loopholes or degeneracies in this system. I'd like to find them and fix them. And I should probably write a proper blog post about this with diagrams since some of this should be easier to convey with images. What I am proposing is similar to some economic systems (mutualism is one of the closest) but I haven't seen this exact thing around weighted ownership.