50 sounds very reasonable. Not only is it plenty long to monetize a work but it may also incentivize aging legends to have a second golden age of creativity at the end of their career.
One of my favorite all time musicians is still touring (and sounding great!) but his entire set list is music he wrote in the 60s and 70s. I suspect he still has it in him to write amazing new music.
We need a lobby pushing to cut it back to 50, which is the international standard.