amateur question, but why would it be a mistake to withdraw from a brokerage account after investing for a year or two?

about to move most (or all) of my home savings into my first taxable brokerage, vanguard, s&p 500. no idea when i'll buy, so looking to gain more interest than my current 3.15% money market.

but i don't understand the tax situation on withdrawals, or the language surrounding it.

example - let's say i invest $500k, earns 10%. after one year, it's $550k. i decide to take out $100k. if i'm paying 28% in taxes at the end of year, that means i'm losing $28k of that... right? so the account would go down to $422k, and i have $100k in hand... but i'm still sitting higher than i would've been with the $500k in the money market for the past year, which would be $415,750 after the 3.15% interest and then taking out $100k.

am i running the numbers right on this?