It can’t be retrieved in one go. But provided every shareholder doesn’t try to liquidate their position simultaneously, then it’s a reasonable metric to use for determining how much shareholder value was lost when Tim was granted the shares.

Disregarding market cap as not real money would be like disregarding your checking account as not real because the bank operates a fractional reserve.

Apple's 2020 revenue was 274 bn $, and on track to substantially increase in 2021. Tim Cook's compensation is about < 0.3% of that revenue.

Then again, I'm not sure it makes more sense to compare stock-based compensation with revenue than with market cap.