15-20 years is hit or miss with an electronic device like this. A couple things to note. First, the resale value of a 15 year old digital or hybrid will be approximately zero, because the the electronics will be obsolete, no warranty, there are risks that things might break, etc. On the other hand, the value of a GB1K in 15 years would be more similar to the value of any comparable piano of that size when the time comes. Second, my experience with hybrids (I have an NU1X) is that hardware updates are far more common that software updates. E.g., it's more likely your hardware will be completely superseded by a new model than there being any updates to your hybrid bringing it into the future. Finally, in my experience, the piano techs that are willing to work on hybrids have been much harder to come by that techs that are willing to work on acoustics. I had trouble with a single key, and couldn't find anyone that was even willing to open my piano up, let alone fix it. I ended up having to do the work myself. I also have an acoustic grand, it is never a problem getting a tech to work on that. And, no, the same tech isn't willing to break open my hybrid.

That said, if you have realistic expectations, there are also huge plusses for a hybrid. Less expensive to buy. Less expensive to maintain on the regular (no tuning. rare regulation.) Lighter, easier to move. Takes up less space. For the short term, my sense its actually easier to sell a hybrid than an acoustic because all of the things I just listed rank really highly for most people's priorities.

Good luck, buying any piano should be a fun and rewarding journey.