Folks still ask for where they find it after 3 years
I think above is generally a valid concern. However some system limitations make it rather slippery to use popularity and historical interest as an ultimate criteria on deciding whether to keep such questions.
Thing is, visual difference between "normal" and historically locked questions is not really as prominent as it may seem to experienced users. I recall seeing at meta references to examples when inexperienced users didn't notice the differences and thought that these are normal, legitimate questions, "Why can't I ask X when Y exists?"
We are essentially forced to decide whether "harm to the Internet" - link rot caused by deletion of particular question - outweighs the harm of it hanging in here and making a broken window for inexperienced users (who sometimes simply can't see / understand what historical lock means).
That's why historical interest alone doesn't determine whether question gets a historical lock.
It's worth noting that so far suggestions to create and maintain some kind of an "outcast" / "museum" site for questions of historical interest were declined by Stack Exchange team.
More generally, all suggestions to help community maintain historical questions known to me were either declined or ignored by SE team. Even when such suggestions require almost no effort and are apparently no risk / no harm, these are ignored (for example setting pink background when rendering historically locked posts proposed here).
(sort of a follow up) Your comments indicate a belief that inappropriate questions are harmless:
need proof that suppressing these questions actually make the site rot, i.e. humorous questions starting to make a trend, because those questions exist in the first place. Without that maybe it's the case of suppressing a harmless thing because of valuing "fear" instead of "evidence"
If you take a closer look at this site history described here you may find out that it is most likely not so.
Thing is, first few years such questions were allowed at Stack Overflow where they coexisted with "boring" programming questions. Fun questions were popular, entertaining, brought a lot of views and site visits. Now ask yourself a question, if these were indeed harmless, why would they want to "outsource" them to a separate site (NPR)?
Nobody in their sane mind would get rid of content that brings views and visits if it is harmless.
I bet Stack Overflow creators would be happy to keep these questions on-site if (if) these weren't causing harm. The very fact that they moved it to a separate site (NPR) makes a very strong indication that these were indeed considered harmful.
In case if you wonder about what specifically could make SO creators believe that there is harm consider taking a look at the canonical article explaining these matters - The Trouble With Popularity:
we discovered that these posts become so popular over time that they truly start to drown out everything else on the site...
Broken windows. Every 'fun' post users see is an open invitation for them to participate in the fun by adding their own fun question or answer. The stuff spreads like kudzu! Pretty soon the entire site is overrun with nothing but that kind of fun. And even if you grandfather a few in, you'll enjoy neverending requests asking why their fun question or answer has to be removed, while this one over here is allowed to remain.
Opportunity cost. Every minute spent participating in an entertaining 'fun' post is time that someone could have spent asking or answering a substantive question, something practical that solves an actual problem for hundreds or thousands of people. Entertainment, within reason, is by no means a bad thing -- but I experience almost physical pain when I think about a brilliant topic expert spending 10 minutes on one of our sites deciding which hilarious cartoon is their favorite.
There is quite a solid evidence that Stack Exchange Q&A model turned out poor in supporting "pure fun" kind of questions. They tried to keep these along with serious questions, they also tried to keep them at separate site... "Sadly, it didn't work out."