Certainly this is much simpler. It's also guaranteed to fluctuate every year. Some years it'll be high, if the market did well. Other years (if you're invested in, say, a 50/50 stock/bond portfolio), you could be stuck with a much lower amount to live on.

In 2022, for example, bonds lost 12.9% and stocks lost 19.4% (https://www.morningstar.com/articles/1131213/just-how-bad-was-2022s-stock-and-bond-market-performance). A 50/50 portfolio would have thus lost about 16.2%.

If your portfolio value coming into 2022 was, say, $2 million, and you withdrew 4%, or $80k, at the end of the year you'd have had about $1.6 million, so in 2023 you'd have needed to live on $64,500.

So, simple, but not at all comfortable.

By contrast, in that scenario your the guradrail approach with say a 5% draw in 2022 would have given you $100k in 2022, and then, since $100k would have been 6.2% of your remaining portfolio, you'd have trimmed 10% and had to live on $90k.

That's a $10k cut from $100k, instead of a $15.5k cut from $80k.