Yeah, I was implying about the 8% growth rate being high, not SWR (second question). I am not really all gloom and doom but US industrial growth was gold in the 20th century and we have a lot more competition now, and I would not want to bank my retirement expecting a certain return. The market also rarely returns at its "expected" rate and that makes short term answers to the retirement question murky. When articles reference long term return rates, there is usually some statistical bias because many articles reference post-WWII to now.
I also wasn't really thinking about how it is used in the spreadsheet. I think it is worth factoring return rate in the calculator to get a retirement date, but as long as you aren't using the result as a hard retirement deadline, I suppose it doesn't matter much. Hitting 4% SWR makes future return largely irrelevant, which is the whole point of the idea of SWR.