Hardly call a PA31 or its ilk a 'heavy' piston twin. However I don't think they're dead. I fly a PA31 in charter in the US, and many other operators use pistons too.
The economics aren't always there for the turbines. Capital outlay, engine overhauls & insurance can be killer. Not so much engine overhauls if they're amortised properly over the TBO, but have one go tits up partway through the period and that throws a large financial spanner in the works.
Single engine turbines aren't always permitted to do what a twin can do: Fly overwater with fewer restrictions. It's a hell of a lot more feasable to operate a piston twin to the Bahamas than a C208 with passengers.