Currently flying the good'ol 76 so I'm biased
although the A/C is 30plus years old, it is still a very productive and dependable platform in its weight range, I anticipate that you will see 76's operating for another 15-20 years. But of course after the teething problems of the 78 and the problems to come for the A350 are ironed out, then the 76 will begin a gradual slope into irrelevance.
I have to say now that I have been flying it for a few years, that I understand why the 75/76 series have such a good reputation amongst pilots, they are very straight forward, easy to fly and dependable machines.