It's a weird spot to be in and I'm still not sure what to do. I've lost a decent amount of money to inflation and opportunity cost. The feeling is as strong as ever but I know that macro trends aren't things anyone can accurately predict.
I know there are better things I could do with the cash in the meantime, but I haven't done the research. I didn't really expect to be sitting on cash for such a long time. Would love to hear anyone's advice.
If you have this feeling now, and have had it for most of the current expansion, you might want to consider the possibility that your gut sense of whether there will be a recession is just broken, and whether you should stop listening to it. Because I guarantee you it's going to be telling you to get out of the market two or three years after the next expansion starts, and who knows how much money you're going to have left on the table by the time you're done with that one.
Compromise: Put your cash in, for instance, Betterment's new 2.69% savings account so that it's at least earning reasonable interest for 2019. CIT online bank is another alternative at 2.4% for over $50k. Both are FDIC insured.
Take 10-30% of your cash holdings, divide by 12 and dollar-cost average your way into the (potentially bulging) market by making identical sized investments every month.
If we see a crash soon you've limited your downside exposure. If we keep growing you've exposed yourself to some upside. Recalibrate in a year or so.
If this is long-term savings, I would invest it all into a balanced portfolio as soon as you can and forget about it - there are many portfolio examples out there that only use a few 'total market'-style ETFs, as an example. I too have sat on cash at times during the last couple decades and it has cost me a lot of returns. Trying to time these things is basically like gambling and it will drive you crazy.
If you plan to use the cash in the short-term or it's emergency savings, I would personally keep it in cash to protect the principal. High yield savings accounts or short-term treasuries will at least provide some inflation protection.
You also obviously know that one ride on the S&P 500 from a relatively low point, will bury such inflationary losses in dramatic fashion. And one bad ride down from these heights could easily cost you the better part of a decade to fully recover. First do no harm, don't lose money; don't chase or try to force returns. Patience plus money is the critical combination to being positioned to pounce on opportunity. Most people lack the cash when the opportunties are plentiful (eg 2009-2010). And you don't need very many big broad hits (like riding the S&P to a triple over six years out of the great recession) across a lifetime, only a few.
> I know there are better things I could do with the cash in the meantime
This deep into an expansion (10 years at that), I'd say that now is exactly the time to hold the line on your patience. The stock market is seeing close to flat earnings growth and its multiple is very high, combined with everything else it certainly appears to be an out-of-gas scenario. We would need to see rather extraordinary earnings growth - and soon - to buffer the earnings multiple this market is carrying. And we're also not seeing macro economic growth like you would want to see, to feel confident in the market moving much higher than it already is. In the late 1990s we were seeing 4-5% growth (18 straight quarters of 4%+ growth in the late 1990s), now 2% is more common. China was an engine for the world economy for ~15 years; that is now over. Where's the next engine? There may not be one in the near future.
A large share of this market climb the last few years has been nothing more than multiple expansion, it's not coming from an organic surge of earnings growth and productivity growth (the tax cuts were a big part of it, which wasn't organic; that adjustment spike is over). I consider this a very dangerous market based on growth rates & multiples (Warren Buffett appears to also hold that belief, based on his net equity selling and refusal to deploy Berkshire's epic $122b in cash; few have consistently navigated such circumstances better than Buffett, whether 1999-2000 or 2007-2008, he has a rigid wiring for it based on the value available to be purchased with capital; his spidey sense is tingling, clearly).
I'd suggest you stick to being patient, wait for a better price vs value environment, and or for another major financial opportunity that comes along in your personal life. Lacking the capital to seize on opportunities when they become abundant, is a truly terrible trap to be stuck in, infinitely worse than the modest inflationary mouse nibbling at your capital now.
Others will note something about not timing the market. Keep in mind this is absolutely not about timing a market. It's about being unwilling to dramatically overpay for what you're buying (eg paying a 30x multiple for zero growth on various blue chips). It's calculating value for price. That is not a matter of timing, it's a matter of deciding what price you're willing to pay for what value you get. Buffett for example isn't a market timer (he points this out at every opportunity, going back many decades); rather, he has fairly strict rules about what he's willing to pay for what value he gets. Being unwilling to overpay is not about timing or guessing, it's about having rules for what you're willing to pay for what you get.
Note that the person you're responding to has quite literally lost lots of money by sitting on cash. And you're saying, "keep sitting on cash!"
Not only that, but you're recommending cash at a time when central banks are creating more cash -- which is precisely the wrong time to hold lots of cash.