It's not delaying but also reducing it significantly - the FIRE person lives on 1/3 a typical income and saves the other two over the course of a compressed career. Typically they retire once they've saved 25X their yearly expenses and then draw down at a rate of 4% a year, putting any excess capital gains towards their portfolios to offset inflation.

So consumption is reduced by 60% over the course of a lifetime, which by most metrics would shrink the economy by a similar amount - of course, most FIRE people also keep being productive after retiring, so it's probably not so straightforward to account for everything.