No doubt piston aircraft are decreasing in popularity and number, being replaced by turboprops. However you do still see a good number of Cirri and Piper Malibus (the piston ones, not the Meridians) being used.
The primary changing dynamic is that turboprops and jets (most notably turboprop singles and currently twin jets) have increased in popularity for new aircraft. What that really means is that these will become more available for used aircraft when they get to that 30-40 year old range like what we're flying now.
The model for owning and flying those aircraft is going to change, specifically as it surrounds engines. For older turbine aircraft, you tend to hit a point where you will end up only replacing engines with used ones from scrap yards, run past TBO doing just hot sections (this varies some depending on the specific engine), and then when the engines are no longer serviceable the plane is worth scrap value. Note that this isn't entirely different from what we see in the piston twin world, but it goes to a different magnitude.
We're already seeing this happen today. In the MU-2 world I see it with some of the older, lower value planes. You also see it happen with King Air C90s (if anything most frequently with those), Cheyennes, etc. While none of these have the entry level purchase price in line with an Aztec or a 310, you do see them crossing over in some areas with 340s, 414s, and 421s. But eventually the Cirri that are trickling down in value are going to take over the price point space that is currently held by older Bonanzas. For the twin world you're mostly SOL, but there are a few Senecas and Barons still being produced, and most people don't want those anymore.
In many ways I'm not entirely disappointed with where things are going. It was a fortuitous set of circumstances that made the MU-2 happen for us when it did, but I had in the business plan to get into an MU-2 in the next few years. It has proven to be cheaper to own and operate for us than the 414, although this wouldn't apply to everyone. I never believed this when Wayne Bower said it, but it's proven true.
We intend on keeping the MU-2 at least until the engine we did an HSI on last year hits its next HSI (so another 1600 hours or so) as it will then be past TBO and the airframe will be coming up on an aging airframe inspection. But I'm also thinking at that point that an earlier Eclipse jet will get to a reasonable acquisition level, or some of the other turboprops.
