Investments fail, there are plenty in the news. Broad market indices, however, don’t fail. There have been numerous bailouts over the previous decades. Why would one assume any future government wouldn’t continue bailouts if all the previous ones did?
When the organization is as big as the U.S. government and has as good credit as the U.S. did in 2008, you can save an awful lot of financial institutions. But if it gets to the point where everybody expects to be bailed out and people start acting accordingly, you can't. Eventually the government ends up falling, as people start realizing that the economy isn't actually working and everybody is just cooking the books with financial transactions.
Government policy makers know this, and their livelihood is dependent upon the continued existence of the government, and so at some point they declare "Nope, bailout is not going to happen this time. You're on your own." At that point, the last group of people who took stupid financial risks are left holding the bag. It's very much like a pyramid scheme: the going is good as long as you can find a greater fool to assume the risk from you, but at some point there are no greater fools, and you find out the greater fool was you.
Isn’t that all the policy makers, old voters, taxpayer funded DB pension funds, etc that depend on broad market equity index fund returns?
Obviously, the system breaks down when it breaks down (when the currency has no purchasing power left to lose), but until then, the entire political apparatus is incentivized to bail out asset prices.
And if that isn’t possible, then the status of your investments/brokerage/bank accounts is going to be the least of your worries, as you will have more immediate concerns about procuring food/energy/shelter/security.
The Crisis of the Third Century, which starts in 235, is where the inflection point between "broadly stable" and "broadly negative" sets in, and the shocks both of the Crisis of the Third Century and the Plague of Justinian are each larger than the shock of the deposing of the last Western Roman Emperor.
However, the point I was trying to make is that Rome's decline sure lasted a very long time indeed.
Additionally, 235 being so important is only obvious in hind-sight. To anyone living in the Empire in 235, I suspect it felt an awful lot like the Year of the Five Emperors (about as far away from 235 as Diocletian is the other direction) and it wasn't until a while later that it became clear that no Septimus Severus type was going to be able to put it all back together quickly.
How long to you have to live, to see a return?
235 to 285 were tumultuous times, civil war and all that goes with it
Famous last words
All economic gains that don't come from real productive activity are Ponzi schemes. Some last orders of magnitude longer than others.