1st off, I'd like to give a good old fashion disclaimer. I have no idea if the 7702 will be offering an upgrade or not. I've not been told a thing. So anything I say from here on out is just a guess as Marantz has not confirmed anything to me or any other dealer. If fact, I'm not so sure Marantz knows yet and that is a sincere statement. It will eventually be a business and resource decision.
I remember what an operational management professor at the UofMN told me 30 years ago: Price, quality, and service. Pick two of the three. Anyone who attempts to deliver all three will be out of business or lose money on that product. When you pay 2X for a product (8802) you have more of a right to expect a higher level of service. In this case, service means options and support. After all, I suppose the 7702 could also offer some sort of HDCP 2.2 upgrade too. But they simply cannot offer this same level of service on the 7702.
We all know that service costs money. When I have a wish list for companies including Marantz, invariably the question always concludes on how they can allocate precious resources. Then let's talk about resources. I do know this. Marantz spent an immense amount of time making Auro 3D work inside of the platform. They are charging for it but as Waboman mentioned, how many people are willing to pay for it? It takes a lot of $200 bills to add up and pay for hundreds of hours of engineering time. I assume that they didn't add Auro3D to break even. I bet if 80% paid for that Auro3D upgrade, they would have offered this DTS:X upgrade. That would be a no-brainer but I bet under 5% bought the upgrade. Heck, maybe only 2-3% bought the Auro3D if my customer base is accurate/representative. To reiterate, I am just thinking like a business person and have absolutely no inside info.
Anyone that I sold a 7702 to I explained not to expect a DTS:X upgrade. Because no promises were made by the manufacture and surely not by me. I promised them that if it was not supported and it was defined that the upgrade was important to them, I would go out of my way to make sure they were very well taken care of in the pending product upgrade/update cycle. But I am guessing only 20% of my early 7702 adopter customer base even has an immersive sound layout. While we are at it, maybe 25% have a 4K capable set. So while D&M pays attention to the early adopters they cannot make business decision based off of 20% of the people who expect DTS:X.
My guess is D&M will make a decision about their resources and figure out what makes the most sense in a business mind set. No one has to worry about DTS:X for many more months because currently it isn't happening for many more months. There will be more technology on the horizon that you will want. So for those who bought or are planning on buying a 7702 enjoy what you have now. By definition, there is never a perfect time to buy a prepro if you place a mandate on having the latest and greatest because you are 6 months away from wanting the next great feature.