I think the big concern (and somewhat legitimate) is the *long* term durability/viability of the electronics-laden tractors being made today. What's a future owner to do 10, 15, or 20 years from now when Deere (or other mfg) lists a complex electronic assembly as "NA"? Certainly there are major mechanical components that when no longer available will sideline a tractor just as badly - but simple things like linkages, bushings, bearings, seals, heck even shafts and gears can all be 're-invented' long after the oem parts have gone away if the will is there. Proprietary electronics, probably not so much.
Maybe I've been too used to seeing 30 and 40 year old (and even older) tractors still doing most of the small farm duty around this part of the country. Small wear parts are there or can often be 'adapted' from industrial suppliers. New tractors (newer than 15 or 20 years) are an exception around here.
While the electronics in today's tractors (where it's not just a bell or whistle but an intrinsic part of the operability) may be perfectly reliable in the short term (first 10 years) - I just don't have a warm and fuzzy that these tractors will have the kind of future sustainability that those 60's and '70's tractors have now in 2005.