The return calculations are a red herring for many younger FIRE people, IMO.
I think the real issue is the assumption that their spending will never change. At the extreme young end of the FIRE discussions the people in their mid-30s have barely been independent adults for 15 years yet are setting fixed budgets for the next 50+ years of their life.
Many of these budgets have obvious blind spots, like the people who paid off their cars before setting up their FIRE budgets, so they forget to include future costs of replacing that car some day. Likewise, many of the frugality stories come from people who were high spenders in the past but recently became frugal, which ignores the fact that much of their current lifestyle (furniture, clothing, electronics, tools and equipment) were funded by their pre-frugality spending and therefore not accounted for in their current budget. Things decay and must be replaced eventually.