Basically this.
I honestly think one way right now would be to close all of the old promo threads that haven't been updated for several months to a year and older. Because people do keep raising those from the dead, unaware that the OG poster and other users on that thread are gone.
Another way would be to somehow modify that pop-up message when making a new thread. You know how it goes "this title is similar to Thread X"? Maybe it can be modified in a way where it WON'T allow you to make a new promo thread if several others are already in existing with the same topic.
But that would also give room to people finding loopholes in the titles or getting bots to close them before they open. And then that could end up being "well, you're silencing users" and what have you.
There's also the fact that...people just don't plain read. They don't look at the OP to see that "yes, this thread is 10 years old and the last known post was 4 years ago. maybe don't post here". They don't look at a promo thread that has 500+ replies and go "wow, a lot of these aren't getting a lot of clicks. And it's already got so many replies. Maybe I'm gonna get buried here" They just post and go.
Can't cure that, sadly.