RSU vs. Mega Backdoor Roth

Recently learned from FAANG friend that many are selling RSUs to fund mega backdoor Roth contributions (e.g. sell RSU to cover living expenses, while diverting portion of paycheck into MBDR). Since Roth gains are tax free for eventual withdrawal, it got me wondering how much better Roth is in terms of tax savings compared to just leaving RSUs to grow after vest.

Within the top 5 FAANG companies, its stock directional trend (up or down) typically correlates well with SP500 index. Most FAANG companies have consistently outperformed SP500 since company inception.

Here is an example of short term simulation, with starting value of $30,000 (post tax amount, RSU sold at vest), and let's assume consistent YoY growth.

From the simple simulation below, it seems index would have to perform very closely to RSU in order to arrive at a similar ending balance outcome. Historically, SP500 averages about 10% YoY.

I know I am probably over simplifying a lot of things here, but assuming the market is bullish, it seems that letting RSU grow and paying capital gains tax is the better option, rather than diverting into MBDR. Am I missing the allure of Mega Backdoor Roth?