The actual ways to use cash are to find high-yield savings accounts (over the past few years some online accounts have hit 2.5-3%, though that's not common), CDs (which again you've had to go with online-only accounts to get decent returns), or some forms of bonds. These offer guaranteed returns (or at least guaranteed not to lose you anything) which is exactly what you want for your e-fund.
Absolutely invest the rest and reap the rewards (or losses, but over a couple decades should be rewards). But it's a good idea to leave the e-fund in a more stable form. Don't be like my colleague in 2008/2009 who went from being worth almost a million to being worth 200k and needing the money. He couldn't let it sit and recoup value, he just lost. His retirement was pushed out years as a result.
Admittedly, his retirement still would've been pushed 1-3 years anyways due to the financial need at the time. But he used almost half his investment's value at that point ($100k), which in a few years would've been back to $500k or so in value. If he'd even had $50k in cash (less than his annual net income), he would've only lost a quarter of his investments instead of half.
It's an emergency fund - likely not much. Maybe a few grand at most in that amount of time?
> and needing the money.
Unless the market cuts in half over-night I'll have the money. That's why I have twice as much as I need in that bucket.
I've not really heard any compelling reason why I should move to a savings account. Yeah I might make more if the market takes an unprecedented long term decline, but realistically I'll make well over twice as much returns as I would that savings account.
As for volatility of the market I accommodate for that by having well over twice as much as I need in investments that I can liquidate.
The end result if I project using historical data is that I'm much wealthier.
If the market crashes to the point people aren't buying S&P500 index funds I highly doubt people with money in the bank are going to be in any better of a position than myself.
If it continues on trend, I'll stand to do better than them though.