we"re still basically flying the same old planes with the same old equipment we've had for decades.
Agreed especially as far as twins are concerned. Though with the advent of Aspen and the likes the cost of "glass" is very comparable to repairing/replacing old style HSI's, RMI's etc with their steam driven remote gyros. So you are seeing some older aircraft being retrofitted.
However the real problem is finding suitable replacement piston twins. While turbines are suitable in many instances there is still a place for piston twins. Hence one reason the fleet of old aircraft.
One other issue is being able to operate at a price to make a buck that allows improvements and upgrades. There's too many who think they can operate aircraft at rates that don't reflect to true cost of operation and cut their throat and everyone else's at the same time.
While not directly related here's an example of what I mean. There's one hero (one who has already lost a a lot of other peoples money in aviation) in the news recently importing and selling a LSA. Apparently it only costs about $50 per hour to run and will change the way pilots are trained and be the choice of recreational pilots. My very conservative estimates based on 200 hours utilisation, presuming you will insure and hangar it, is $115 per hour at the very least.
No wonder when we have idiots promoting ideas like this that we still have old fixed card ADF's and other such fantastic gear, as many do not charge what they should to operate their aircraft.