The current "lifetime of the author + X years" rules in effect in the United States still carry the same perverse incentive, though the incentive diminishes rapidly as X gets larger; with the very large value of X in effect today the perverse incentive is so small as to be effectively non-existent, but it's still there in theory.

Personally, I'd prefer a fixed term. I know enough independent authors making a living from selling their books that I'm willing to allow the fixed term to be large, like 50 years from date of completion of the work. (With a good definition of "completion" so someone can't cheat by editing a couple lines per year to keep something copyrighted indefinitely). The simpler the rule is, the easier it is to understand, and the harder it is to cheat it. The more complicated you make a rule, the more loopholes get found.

There are significant perverse incentives for me shooting you with a gun and taking your money and running away too. It mostly doesn't happen.