Of course the whole world could go into a multi-decades-long recession, but then we’ll have much more serious problems anyway.
If the world did go into a multi-decade recession, what “more serious” problems would you have then your investments doing poorly?
You might answer things like “ buying food due to shortages” or something, but surely whatever problem you name, being more rich is going to solve it?
Now you can invest on the thesis that this isn’t going to happen, but to argue that the whole concept of investment is useless if it does seems very suspect to me.
Being rich only matters as long as your investments/assets hold any value. If truly serious problems around your investments go to 0, your assets are only worth something as long as you can maintain control of them (police won't be around, nor will judges be) and even then your car will be worthless without gas.
It all depends on what meaning a person assigns to "serious" in this context. Personally as long as being rich solves my Problems I wouldn't describe any situation as serious.
Also, as long as poorer people are not after you and your properties (and your life, even) through a revolution, which revolution could be caused by world-wide economical and societal crisis (if not a revolution then maybe a civil-war where the rich are of the wrong ethnicity etc)