I've used a K780 for the last 3 years and replaced it with a K380 a few months ago, I use it 8 to 10 hours a day 7 days a week, I've recommended the keyboard along with your mouse (mx ergo myself , and MX vertical for my wife) and I was proud a lot of my coworkers followed and thanked my example of buying this combination, and now you have removed 50% of the advantages of this by removing the use of the "fn" button combined with the mouse buttons, I've been trying to substitute this with other configurations but I just cant figure it out. It brought bak to my memory the old HP saying: if it aint broken DONT FIX IT!
I would suggest you consider bringing that combination back, not sure how many comments you've got on this but I really miss it, and it was one of the main reasons I've bought your devices the last years