"Electronic components have doubled the possibilities for incompatibility. They have brought us not only incompatibilities based on intended use similar to those of mechanical systems, but also incompatibilities that we constantly run up against with our computers due to software revisions. When it comes to Di2, we can lose compatibility with components that had been happily working together by simply performing a software update."
Isn't that just the greatest news? Aren't you just rushing out to the bike shop to upgrade all your so-last-century mechanical derailleurs for this Brave New World of electronics now? I mean, in the past, you just had to worry about hardware compatibility. If your stuff wore out or broke, it could be difficult to source a compatible replacement as Shimano and Campy chased the never-ending "n+1 cogs" dragon. But as hard as they tried, they couldn't make an existing, functional, mechanical drivetrain simply stop working without sending ninja mechanics into your garage at night to tweak your bike without your consent.
But now? Now our bikes can be just as reliable and future-proof as our computers and phones! Today's perfectly functional hardware can be turned into tomorrow's paperweight with the click of a mouse! "Oh, I'm sorry, sir... Derailleur Operating System 2.3 is only compatible with derailleurs that have an Intel processor. Your derailleur can still run on Derailleur Operating System 2.0.1 in compatibility mode, but it will only shift three gears and is not officially supported by the manufacturer. You probably just need to buy a new bike."
My only (futile) hope is that the software running these things could somehow be hacked and/or made open-source. Then maybe a code whiz could figure out a way to make "officially" incompatible things play nicely together. Di2 on a five-speed rear wheel, anyone? Maybe shifting a SRAM 1:1 rear derailleur? Or a Di-2-controlled Rohloff hub? Now that could be fun...