Piston twins are now not worth much more than engine value. Its getting very hard to find good low time "working" twins (eg PA31). Many corporates are now mandating turbines.
The Charter market will always prefer twins for the image of the second engine. King Airs are currently taking over from Chieftans. The freight market will move to single turbines (eg Caravan). Tourist operators will use what makes sense to them (C210, C206 Airvan, etc). The piston twin will become the domain of dinosaurs like me who like the speed & security of a twin for IFR and use the aeroplane for business a little. Twins with good support (Seneca , Baron, Aerostar) will have a good future. The others (especially SID requirement Cessna's) will fade away. You can see this happening with C310 prices now. Chieftan parts are getting hard to get & expensive now and the source of good low time ones (for fleet rejuvenation) has all but dried up.
But I could be wrong.