The industry standard DESIGN lifespan if a general use piano has been 30-40 years for a very long time. (A high use piano gad a shorter design lifespan). The designers/makers know that under normal operating conditions, it is in that time frame that critical components are in enough state of decay (normally) that the instrument can no longer function at its prime. Their recommendation is to rebuild it (totally or partially) or preferably replace it. Ultimately it depends on the use to which it is put and the environment in which it is kept and the care it receives.
Ask any manufacturer of any item...car, truck, refrigerator, lamp, chainsaw, battery, etc etc. They all have a design lifespan with normal use. Add abnormal use or neglect and the lifespan drops. Add regular routine maintenance and/or obsessive detail maintenance and the lifespan can go up, sometimes dramatically.
The sales depts on the other hand are in the position to SELL. The more they sell, the more they earn, the more get made, etc. Unfortunately sales people are notorious for saying anything to make a sale. (This is universal, though there are honest sales people, even they have a tendency to fudge the numbers a bit). It is the PR end of the piano industry that is responsible for the myth of multi-generation performance claims. They can do this BECAUSE the average buyer has no idea/knowledge about what they are buying and can be easily swayed. Also, sales people are rarely (if ever) held responsible for the claims they make. Once you sign...its your decision, it's now your responsibility.
SS is possibly the biggest culprit in this whole scheme from way back. But since it works to sell pianos, all the others jumped on board too. Therefore the myth spread exponentially. They claimed (and continue to claim) that buying a SS piano is a better investment than the stock market. Ha ha ha! (They're not that far from the Brooklyn Bridge either).
So, the first 20 years are probably the best years (if maintained properly according to the usage it is put). It's not going to "last" 100 years. It may "exist" but not "last" if you understand the difference.
Peter Grey Piano Doctor