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The advantage to my eyes is that you could do 8-speed electronic
Who would want to? The kind of retrogrouches who want 8-speed are the same ones who still want their mechanical gear shifters (non-indexed, obvs.) on the down tube. I'm guessing well over half the people buying bikes with electric gears wouldn't even be able to change their own chain, so the nuances of backwards compatibility are unlikely to be their highest priority. Most of them are not weirdos like us, they are cycling enthusiasts rather than bike enthusiasts - the machine is a means, not an end, something for other people to fix and to be replaced frequently.
gbj_tester-
Granted, that was a silly example. The point is you can do any speed within reason, which sets you up for 11-, 12- and 13-speed (which is also on the way), as long as you have a derailleur with the a suitable parallelogram and cage length*, and allows you to upgrade bits as and when you see fit. If I buy Shimano 11-speed Di2 now I am committing future me to throw the whole lot out next time I want to upgrade (or sell it on eBay).
I just think it's going to work out cheaper and more versatile by far in the long run, once someone does it.
* folks are going to be on 11-30something doubles for a while I think, so there should be plenty of derailleurs around for that
Edit: anyway I'll quit banging on about this now. Maybe I should build one myself and keep my mouth shut until then
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I'm in the weird bit of the Venn that couldn't give much of a toss about the bike but definitely wants Di2 and a powermeter and can replace a chain. Mummy always said I was special...
frankenbike
hippy
The advantage to my eyes is that you could do 8-speed electronic, or upgrade your 11-speed stuff to 12-speed with just a new cassette and chain, or replace your derailleur for £20 instead of £100, or whatever.
If you've got 10-speed Di2 and want to switch to 11-speed you have to buy a whole new bunch of parts. IMO that is because of planned obsolescence and greed and nothing else; I'm 99% sure that they could have upgraded them to 11-speed through firmware alone. In fact they used to work on 11-speed before Shimano disabled it. How likely is it that current 11-speed eTap is going to work with 12-speed stuff? Not a chance in hell.