I agree with your general idea here but could it not be argued that pensions are just an abstraction on top of 401ks in the sense that if the demographics are shifting such that non-producers outweigh producers then you would expect the economy and business profits to suffer. These effects would cause broad slow growth or even reductions in business valuations, and therefore poor performance in investment accounts broadly.
Yes, and I think that describes the past few decades in Japan (among other factors).
Other options included pensioners taking a haircut and/or workers taking a paycut, neither of which is particularly palatable and still runs us into the ground eventually. Increasing corporate taxes could square the circle for a little while, but then corporations would slowly move more operations offshore, decreasing tax revenues eventually anyway. There's also the option of cutting other government expenditures, but eventually that stops working and at least some of the things that were cut can't stay un(der)funded forever.
The only other (distantly) feasible idea I can think of would be, again with an eye to Japan (of the past), near-total isolation from other countries economically, but that would have an even more substantial stagnating effect.
