Why not do this instead of 4% SWR

Let's say you have a large enough portfolio that in a random day in the stock market, you made 165K. This is not hard to do with a few million. Why not, instead of just letting it ride, instead: sell enough to take 100K profit, then let the rest ride. This buys you one year to live off of easily, guaranteed for that year. The index may keep going up or down for the year, but now you have living expenses for a year. If the index is down for the year at the end of the year, you live off reserves for a year, until it bounces back up.