You'll find a lot of people trapped in homes they cannot sell. The cost to update very old homes would far exceed what most people can spend out of pocket, and by the time they paid off the debt to do so, the codes may have changed again.

Also, older couples who have lived in one place many years and are now retired generally cannot take out lots of money. Instead of being able to sell and move to a senior community or nursing home, they would be stuck with a property they cannot maintain, compounding the problem.

Essentially, you would be hurting vulnerable people at the advantage of the young and wealthy.

If the updates were mandated as the code changed:

See all of the above, but now you're forcing people out of their homes even faster.