Redned is right about "ticking the boxes" - when I was looking for a replacement bike for touring duty the only bike that ticked enough of my boxes was the F800GT (specifically, low seat height, light weight, low center of gravity, integrated luggage/passenger accomodations). These were significant enough that I bought the F800GT despite it being a BMW (sorry, I never drank the BMW Kool-aid - cost to buy, cost of service, and reliability were all big negatives compared to the Japanese competition). I also bought it despite it having the single most uninteresting motor of any bike I've had (gutless, and a pitiful sound compared to my 2015 Triumph Bonneville with a similar parallel twin motor that sounds SO GOOD). A "bonus" was the maintenance-free belt drive but that was offset by the ridiculous price of a replacement so I never considered it a plus, just a small bonus. Still, no competitor on the market could touch the "Big 3 items" of low seat, low weight, low center of gravity, and passenger accomodations.
But I always do the research to see what I would buy if a replacement car or motorcycle was ever needed quickly, so comparing those ticked boxes to the F850GT ... well, there is one bonus as the motor may finally have some guts down low and finally sound decent too. There's no change with the integrated luggage; unknown are passenger accomodations but they should probably be okay; but the big losers are it's heavier, the fuel is now carried higher on the bike likely affecting the center of gravity, and it carries LESS fuel than the too-small tank of the F800GT (if you are gonna call it a "touring bike" it needs a bigger tank and 250-mile range, IMHO). If it turns out that the larger, more powerful motor is also more thirsty then the smaller fuel tank will be a huge negative. So for me there is no "great leap forward" from the F800GT, and in fact it may not even be a break-even deal. I see no reason to trade my F800 for one.
But again, I have to think about what I'd do if I HAD to replace the F800GT. Well, now there is serious competition from the Yamaha Tracer 900GT - even BMW seems to agree that is the standard bearer in light touring bikes. Looking at my boxes, it's as lightweight as the F800GT and lighter than the F850; it has integrated luggage, a larger fuel capacity (but real-world fuel mileage is unknown - it may go no farther than the F800), it has a GREAT motor, and Yamaha reliability. Center of gravity and passenger accomodations are unknown and would need a test ride. And it's a bit costly, though likely priced much lower than the F850GT will be once they are loaded up with packages by the dealers. It's looking like a winner here. Oh yeah, and seat height is ... WTF??? 33.5 inches or 34.1 inches??? And in one fell swoop the Tracer is completely eliminated from my consideration.
So if I trash my F800GT tomorrow and need a replacement then yes, the F850GT would have to be considered. But based on the success with my 2015 Bonneville on a recent 5-day tour (despite not having waterproof hard luggage, heated grips, or wind protection) I'd more likely get a new Triumph Speed Twin with its excellent motor and low seat and set it up with some aftermarket hard luggage and heated grips as a touring bike (and get an aerostitch suit for weather protection) leaving my beloved 2015 Bonnie with its proper air-cooled motor for local excursions. So in the end BMW may have brought me into the "club" with the F800 because it was "as good as I could get" at the time, but it's followup model likely means I'll not own another BMW in the future (except maybe an old airhead). Of course, one has to wonder if an F850 motor would bolt into the F800 frame ... hmmmm ...