But there is nothing theoretical about start up equity in companies that have raised money. They have a very clear valuation, and especially Series D, that valuation will already be pretty mature.
AirBNB did a Series D with a valuation at 10.5 billion in 2014. They are currently around 91.66 billion market cap. That is a nice 900% return, but that's not a completely different ballpark than other LargeCo's (Apple had about 800% return, Amazon 700%, Meta 500%) - the question is, do you really think your Series D start up is going to be as successful as AirBNB (which is among the top 100 biggest American companies by market cap).
The point is, there is really nothing "theoretical" about the value of those start up shares - they have a well determined value, and for many start ups, even the appreciation of those shares between Series D and IPO can be less than the appreciation they would have gotten from LargeCo shares with top of market compensation and potentially even bigger dollar value equity stakes.