AFAIK the GNS boxes are screwed by
The first can be re-engineered for a different display (in volume, it would be a custom TFT unit anyway) although apparently this is what initially killed off the KLN94 (and HBK lost all their engineering capability c. 2003). The second is harder; again possible but since they have the GTN they prob99 decided to move on.
I don’t see how this is any different than all the other old stuff we have on our decades-old airplanes, including many planes which have parts one can no longer obtain. In the case of the GNS we have Garmin who at least stopped upgrading old units to preserve parts supply, as well as gazillions of units produced which can be harvested for parts if needed. There are plenty of third party shops doing refurbs, just as there are shops refurbishing decades-old King systems. Given that loads of airplanes are still to be found with much older bits, I still bet the GNS lives on for quite some years.
Of course, one can keep 430 as there are plenty around for repair & replacement but be aware the cost to repair GNS430 will exceed the cost to buy and install GTN650
If you ever ask how much it cost to maintain or replace an old BK HSI & KNS80, I would say possible but likely all the money you have 
Ibra wrote:
If you ever ask how much it cost to maintain or replace an old BK HSI & KNS80, I would say possible but likely all the money you have
Well I just did this, today in fact. My KI-525A returned from Termikas non-functional. It was €1250 for a refurbished drop-in replacement with no aircraft downtime or €7k for a GI-275 with unknown delivery date. I opted for the former.
Indeed; you can pick up used KCS-55 components very cheaply nowadays. Some % won’t be working but even if you buy two, it is cheap
Back on the topic
750 is much better value than the 650. Not only for the moving map, but it’s much easier to operate (touch screen likes larger sizes), provides more convenience (e.g. when searching for a frequency you see a longer list and not only 3 lines at a time) and finally providers much higher safety (e.g. by showing a preview of an approach you select before you do so…)
Malibuflyer wrote:
providers much higher safety (e.g. by showing a preview of an approach you select before you do so…)
That is certainly more convenient, but I don’t know that it adds much to safety as you anyhow have to compare each approach leg in the database to the approach plate. Ref. AMC2 to NCO.OP.116 item (c)(4).
In my opinion it does as there is another layer of checks. It happened to me more than once that after selecting an approach I immediately saw that “something is wrong” because it graphically looked wrong.
Yes: Most probably I would have realized this also when checking each leg. But in my opinion it is undisputed in aviation that an additional layer of redundancy creates additional safety.
Malabuflyer.
That is a sweeping statement of cost vs functions. The 750 is worthless to those without the panel space to fit it and it’s functionality advantages disappear if the aircraft is fitted with a large PFD/ ND.
For the panel space the 650 has more functionality albeit a little more difficult to access , I could have fitted a 750 but 2 × 650’s and a G500 provide more redundancy and avoid an audio nightmare should the 750 screen fail.
The situation an owner finds them selves in when upgrading a panel makes best value a moving target that can’t be defined in a sweeping statement , but is defined by the legacy systems and panel space.
