I'm a bike nerd, so it's interesting to me and I can see more point on road bikes than mtbs (since road bike shifting is so compromised with flappy levers, cable routing designed to cause the cables to bind, etc) but cables are a good solid solution.
7900 was compromised (as is every other shimano groupset of that generation that followed), but 7800 is brilliant, and by all accounts 9000 is as good/better.
Not sure about auto changing to maintain cadence: 1) I like to vary my cadence - what if you're out of the saddle?! 2) I'm not sure random unexpected gear changes would be much fun! 3) Off road particularly your cadence changes constantly, how do you either have it keep up, or not change 5 seconds too late
Not insurmountable though. keep the shift buttons and use them to vary cadence instead, click down and it drops 5rpm before you stand up to avoid drop kicking the guy behind, then a saddle sensor to automaticaly go back to your normal cadence when you sit down again?
Would also need an input from the brakes/speed/gradient too, to stop it shifting into a really easy gear when coasting downhill or upto trafic lights. TBH it's probably far too complicated to work well for everyone. It'd be a cool 'mode' for a training session (maintiain 90rpm for an hour and let the computer pick gears that make you do a 2x20min FTP workout regardless of headwinds, accelerations and gradient). But for just riding arround, races or club runs, 99% of people probbaly just flick gears subconciously based on 101 factors a computer program would struggle to resolve, how would it know you're about to aproach a climb, even with a GPS/map data to base it's decision on you might want to either ease off in anticipation of an attack, or up the speed to get to the front in anticipation of being dropped (basicly Cav and Froome would want entirely different modes).