With RSU's, you get $400k1.05^4 (4 years of compounded growth)
With cash, assuming you immediately invest the money, you get $100k1.05^4 + $100k1.05^3 + $100k1.05^2 + $100k *1.05
Running those numbers, the RSU's are worth $486,202 at the end and the cash is worth $452563. RSU's appreciated by $86k over the duration, cash appreciated $52k over the duration.
It's the time value of money. Getting it earlier makes it worth more.
Of course most people won't do it because is very risky. Yet getting RSUs has similar risk (or larger as you can lose more than the option premium).
There might be US tax implications that I'm not familiar with of course.
Also I'm not familiar with the US case, but I understand those limitations are contractual not regulatory and thus have no bearing in what's would be optimal for the employee.
If, along those 4 years, your company tanks 25% (Shopify tanked over 50%), you’ll be able to abandon the investment (and get 100k a year of something else), or double down and get more shares (aka dollar cost averaging).