I'm trying to get my head around why Garmin would hold back development of NXi upgrades for Diamond aircraft. They make a big deal of
upgrade availability for other models, going as far as to say it's 'a virtual “plug-and-play” swapout for your current displays'. Of course that is an over-simplification, and I'm sure that a non-trivial amount of support from the manufacturer is required to make upgrades available for specific models. But I don't see any reason that Garmin would try to get in the way of such an upgrade path. What would be their motivation to do so when upgrades provide them a means to sell new components?
If a "lowly" Cessna 172 can upgrade from G1000 to NXi, why not a DA40-XL? Even a "dead-end Phase I" upgrade provides access to a better UX for the current set of features as well as a perception that customers haven't been entirely abandoned. Whether that is worth $35k+ ... the market will decide.
On the flip side, I can think of several reasons why Diamond would drag their feet on upgrading old models: lack of staffing or technical expertise to support more configurations, no guaranteed monetary positive return on the investment required to make the upgrades available, risk of introducing new problems into functional aircraft, peception that it may cannibalize sales of new aircraft, and a possibly-poor relationship with TC resulting in higher hurdles to getting approvals.
Is the blame being assigned to Garmin due to them not making upgrades profitable for the manufactuers? I could certainly see that as being a factor. Diamond has to invest some resources into making upgrades available, and I doubt Garmin is willing to subsidize those costs. So the costs would either need to be recaptured by increasing the upgrade pricing (where there is perhaps not much wiggle room) or through the harder-to-measure increase in customer goodwill/loyalty, which Diamond has historically valued somewhere close to zero.
It will be interesting to watch what Cirrus does here since they have a much larger installed base who would benefit from these types of upgrades. I don't see Diamond taking the lead here, but they may follow if Cirrus steps up like Textron has. Meanwhile, it seems like the best we can do is let Diamond know that there is some demand by contacting them directly as suggested by Paul above.