> When you're investing for retirement in your 20s or 30s you can wait out a 5 or 10 year downturn in your index funds.
Sure, I know all that.
But the point, or rather the question, remains ... what are you investing ?
Which brings us back to "don't invest what you can't afford to loose", because by definition you can't afford to loose that for days or weeks, let alone 10 years !
You should really have 6–12 months worth of salary in hard cash in bank accounts. Then you only start "investing" whatever spare you have after that.
Why 6–12 months ? Because bad things have a habit of coming in pairs. You have an emergency that needs ca$h, you have a problem with your house that needs ca$h, you need to pay your mortgage with ca$h ..... and then your boss comes along and tells you that there's no job for you anymore.
6–12 months might sounds like an unreasonable amount, but if you lost your job tomorrow, nobody is going to be generous and wait for you to find another job, the bills will keep on coming.
Most people also don't have an ability to walk from one job to another. Realistically it will take (at least !) a couple of months to find another job if your departure from your previous one was unplanned.
Cash-in-the-bank provides you with a safe number that is a close to guaranteed as you can get. Meanwhile you cannot rely on selling off your passive index holdings to support you because the markets could drop 40% tomorrow without warning just because some dictator on the other side of the world got out the wrong side of bed and invaded the country next door.