Yes, this is the elephant in the room. Inflation.Retiring at 39 with $500K is a pretty good trick, IMO. I don't know how you do that, if you are going to live awhile. Prices continue to go up. $50K will likely be needed for house and car repairs alone...
If your job is the problem, perhaps look into doing something else?
Lets say you can live off your $500K investment - lets call it $35K - 7%. For those not paying attention, CPI over the last 6 months is back up near 4% annualized. So lets say we average 4% for the next decade - many say we are. In 10 years your $500K now has the purchasing power of $337K today.
IMHO - if you want to live off of interest / dividends without touching principal - you need to plan for maybe 2 to 3% max real return - ie after inflation - so you can keep increasing your nest egg by the inflation rate.