I'm sorry, what sub is this? I thought it was financial independence but this thread (and other recent "victory" threads) read more like the comments section on one of those public interviews MMM gives. "It's impossible to retire on so little!" "How can you pay for healthcare?" "Well it might've worked in the last 50 years but there's no way the market performs so well in the future." Sounds like a serious brush up is needed on the frequently complained questions.
Yes, /u/swozey OP could keep working and grow his $1mm into $5mm. Yes, /u/Poemi there's not 100% certainty that OP's $1mm will survive for another 50 years. However, there IS 100% certainty that OP and his wife are going to die in the next 50(ish) years (sorry OP). It might not even be that long. What if OP or his wife dies in dies in an accident or from a fluke disease at age 40? What if an unthinkable natural disaster - an asteroid impact, a supervolcano eruption, whatever - wipes out the economy as we know it? Should we just keep working forever because it's impossible to have a 100%, guaranteed to work, ironclad surety that our money will be sufficient? Of course not! Besides, quitting isn't permanent - OP and his wife are both credentialed and experienced workers, and there's nothing prohibiting them from re-entering the workforce if things don't work out, or even earning part-time money during ER.
FI is all about balancing two finite resources - time and money. There is no scenario that will give you 100% certainty that both of those resources will last as long as you want them to. The best we can do is to understand the probabilities based on what has happened before and make a reasonable plan from there. OP and his wife have saved a reasonable amount of money based on their modeling and expense needs. If you feel you need to save double what the calculators say to meet that "reasonable" standard, then more power to you. But don't begrudge OP for finding his own balance point.
OP, my congratulations. I hope to be joining you in the ER club one day soon.