> I understand why that was done in the company’s early days, but there ought to be a better way to reward/incentivize early employees that doesn’t rely on the fickle and myopic nature of publicly-traded stock.
The IPO is the carrot that you're dangling ahead of early employees many years prior to getting to that point.
When you've gotten to a state when you're ready for an IPO, they are expecting to actually get that carrot. You have no leverage or new incentives that you can give to an early employee after you IPO. If they are sticking around after, you're either drowning them in money, or they are doing it as a courtesy.
So, your alternatives are to ask them nicely, or give them a boatload more money to stick around.
> They had an almost endless supply of interested private investors pouring in money regularly
It sounds like they were interested in pouring money in to grow the business, not to reward early employees. (Which is perfectly reasonable.)
This is why you're finding the two things at odds with eachother. As an employee in a pre-IPO company you have a much smaller small amount of leverage for any sweat equity you put in, compared to someone who paid real dollars for their equity. Your interests aren't aligned with your investors, and their interests aren't really aligned with yours, outside of one thing - you both want to get to a point where you can cash out, via IPO.