I don't have a crystal ball, but I have read a lot of studies on US stock returns over the last several decades.
Its clear that something like index investing (buying the whole market) is a no-brainer stupid way to go, and beats inflation by ~5% per year long term, in effect doubling your investment in real terms in like 15 years or so. On average.
People who try to time the market do worse than buy and hold. Consistently. People that do stock picking (without timely real technical or insider info) also lose money relative to buy and hold. Consistently.
The conundrum is 'how can the market go up and up forever?' A very reasonable question. Stocks go up because the economy grows faster than inflation, due to higher per capita money AND higher productivity AND due to population growth. Those factors together get you close to the 5% real figure (above) long-term.
In practice, the (indexed) market between recessions goes up more like 8% real on average, doubling in real terms in about 5-6 years rather than 15 years! This is NOT sustainable.
Corrections (>10% drops) happen a lot, for a lot of reasons, and are impossible to time, and they are short lived (like months) so they are easy to survive and pass quickly. Corrections bounce back, even severe ones like 1987.
The major issue is genuine recessions. The major drops during real recessions reset those 8% gains to 5% long-term gains. If you invest a lump sum right before a recession drop, it will take you years to get back your money. If you retire based on assets valued right before a recession, then maybe you can not afford to be retired like you thought, and take years to catch up or go broke in an edge case.
If you could time just the recession drops, you COULD make 8% returns forever, but this is very hard. That is the only crystal ball you need or want.
As for the present, yeah, valuations look high, esp in tech. Productivity is super high right now. Demographics in the US look great. Both of these factors support the 5% base growth rate, and were not the case btw 10-15 years ago. We will certainly have little bumps with COVID waves (as for the 18 mos, during the build up of the wave), and corrections can ALWAYS happen.
But the only question is will there be a true recession. Buying and job creation are through the roof (which looks like we are super far from a recession), but sentiment is nearly as bad as 2009 (which suggests were are in a recession). So which do you believe more? I happen to think the sentiment suppression is politics- (and thus media-) driven (on both sides of the aisle) and the economy is booming.
Ofc a lot of recessions are triggered by spikes in oil prices. But I don't see that happening here, since it looks like oil is already dropping (ofc it could spike again, who knows) AND bc the share of the economy going to pay for oil is tiny compared to decades ago.
I'm not saying the economy is booming at a level to justify current valuations, but enough that a recession is nowhere in sight. Risk on, and enjoy the 8% returns (and don't sweat the dips).