Is it my imagination, or are there fewer problems with sprag clutches and HES sensors the past couple years? 

Seems at one time there was at least one active thread dealing with a slipping sprag or a bike that died after 20-25 miles.  I can't recall more than a couple threads in the past couple years.  Are owners more diligent with servicing their oil now?  Have we weeded out the marginal HES units?

Seems to be fewer throttle body bushing leaks as well, the only consistent problem seems to be with the old Z tube and fuel systems. 

Are these old beast falling into the hands of owners who take better care of them now?

Better approach to oil changes. But I also think a lot of the older bikes are now landing with caring owners.

I have Ks for 13 years and now regard Z pipes, vacuum caps, vacuum hoses etc as service items.

As an example my K100RS has been acting up lately. It bugged me because last April I replaced the lot before a trip to France. But it was showing usual running too rich problem. Yesterday polishing it for the purpose of checking over everything I noticed the vacuum pipe was cracked. I was happy enough to see that. So today as a short term fix I used tape over the pipe to seal it off. Fired it up and she ticked over nice and sweet at 900rpm. Now to actually replace the pipe. Easy now since the lot was out in April and copper grease is everywhere.

Better oils with better cleaning additives may also help.

I now do a 50/50 mix of Mobil 1 15-50 and Mobil 1 High Mileage 10-40. The HM oil has additives for cleaning as well as seal reconditioning. My theory(hope) is that the HM oil additives will keep the sprag clean and also help keep the water/oil pump seal and rear main seal from failing.

Yes, I think that better additive packages and better owners may be a big factor.

Vehicles(autos, motos, and boats) go through life cycles where they are new and well cared for through pride of ownership, old age where they are depreciated and functional with owners who do just enough to keep them running, and then on to the vintage/classic status where they have owners who are to varying extents emotionally tied to them, or possibly look at them as investments, and then take better care of them.

As far as the electronics, so far, except for temperature sensors and HES, there haven't been that many problems reported here. Jetronic, Motronic, Ignition controls, and ABS brains so far seem to be pretty rugged.

However, I recall from my industrial control days that the electronics used have time to failure curves where a long period of low failures is followed by a period where most of the failures occur and then the failure rate drops off again.  Sort of a bell curve.  I wonder when we hit that peak and how big it will be?

Hopefully not your BIG BEN size more the normal BELL/END size

According to me, it's less an improved quality of electronics or lubricants which explains this than the fact that the periodical and temporarly fashion for K vanished, and fewer people ride them...
But btw, I had to change HES on my K1 past year!

You're correct about the failure curve for electronics.  There is a high rate of failure right at the beginning of the life of the device where early life failures occur.  Then the curve is low and flat throughout the expected life of the device, the MeanTime Between Failure number, and then a return to very high failure rates as the lifetime is reached or exceeded.  With 30+ year old electronics in these motorcycles, I'd expect to see a rise in the failure rate soon. I should think that electrolytic and tantalum capacitors would have caused havoc by now but that's not the case.   It is.a testament to the build quality that these complex electronic devices on the K-bikes.