dropper posts for drop-bar bikes are already available. I can see these becoming popular for a certain segment of the road-going population who rides routes like the one you describe, but I think most roadies will not bother with them. I use a dropper on my mountain bike (hardtail) and I'm 5'9", but I can't see any use for it on the terrain I ride on my "gravel" road bike, even on singletrack. maybe I'd grow to appreciate it if I had one, but it's doubtful that it would be a game-changer like it is on my mountain bike.
the other segment I see is urban bikes, especially for shorter riders. when I got my wife, who is quite short, to try out a mountain bike with a dropper post, she was super excited about the opportunity to drop the saddle for mounting and dismounting the bike. PNW is marketing the
Coast seatpost to this segment.