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it is possible to adjust that through firmware alone
No, it isn't. The derailleurs have internal hardware in the form of rotary encoders which define either the number of gears (for the rear) or the intermediate indexing points (for the front). That's why the FD6770 could be used (with the old firmware) with an RD6870 or RD9070 rear derailleur, but the trimming function would stop working because the indexing points were no longer correct. That's why Shimano blocked out RD6870/FD6770 mismatches using a firmware update, because although it would work to a degree it didn't work very well, and Shimano don't want people fucking up Di2 by using less than optimal configurations.
Anyone wanting to avoid horrific waste could always stick with 10 speed throughout.
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Brommers
@frankenbike
But the point is why should you have to replace two derailleurs to just to fit an 11-speed cassette? One of the cornerstones of electronic groupsets is basically the superior indexing; it is possible to adjust that through firmware alone. Mechanically those derailleurs will be good for at least another decade. It's horrifically wasteful to replace the whole thing just to change the indexing.