Not a smart one.
Say an average joe gets talked into investing into BTC. Encrypts it on a drive and puts it in a lock box, Saving it for 20 years to retire, buy a new car or house, or even their childs college/car.(Considering they believe the price of BTC's will raise to a certain amount at this point in there life.) They go back after all this time pop it in a computer(Of the future, ooooo ahhhh! lol) And boom, Bitcoin says there data is to old. They depended on that as a back up plan and now it's absolutely worthless. Whose to take from all of us today, and throw it out in 20 years?
JS.
The 20 year rule would be common knowledge. With commonly disseminated knowledge about the rule and an easy way to avoid the 20 year rule, I don't see any problem with expecting people to use the system properly.
Do you also think it's a problem that if you delete your private key file, you lose all of your bitcoins? What about the average joe that saves for 20 years and then accidentally deletes or loses their private key? Does the fact that this can happen mean that bitcoin itself should not exist, just like your assertion that the fact that a 20 year rule should not exist for the same reason (i.e. someone know knows, or should know the rules, still makes a mistake and loses their money)?