Interesting comment and I understand what your saying about musical instruments and thier age, but surely it depends on the quality of the instrument in the first place? For example, if you bought an average sounding keyboard today, then it will show its age far quicker, probably within a couple of years. Because top of the range keyboards have a higher sampling rate, the instruments are going to sound life like, and if thats the case then how much better can a sound of a synth, piano, sax, flute ever be? It will just be variations of the sounds that will change, but if the quality of the intrument is good in the first place then it wont matter how old the thing is in years to come.
My sister still has a PSR500 in which sounds awful (but i remember it sounding amazing for its day)!
But with top keyboards of yesterday & today, they are the ones that could last 10's of years, eg, X1, PSR9000, KN6000, VA7 type instruments. In the past, the PSR5700, Korg M1, GEM WX2 all still sound great.
I have an X1, but I also have a GEM WX2 which is 7 years old (Dec 1993 it arrived). I really do believe that the WX2 can still out-perform some keyboards and match some of the top ones because it has such wonderful sounds and styles, not to mention an unprecidented ability to play songs and SMF. This is a 7 year old keyboard as well.
A good friend of mine still has a Korg M1 in which sounds great!
So i think it depends on how good the instrument is in the first place.