I dont think that Mintpal kills Coins.
It is just that Mintpal usually adds Coins before Major Announcements/Releases what drives the rising price.
So Coin development slows down after MP addition because MP adds on the peak of activity.
There is no reason to worry about Mintpal when developers keep beeing active (what is almost impossible) to hold the Volume up
But for example the Vericoin & Nautiluscoin developers managed to do so