The problem with a question like this is that it's asking to predict the future.
We can reason based upon our experience with the components that are available today but new ideas are coming up all the time. Who knows where that's going to go. My prediction is that the bike of the future will have one front and one rear sprocket and a belt drive. How it provides for different gear ratios I have no idea.
Yeah and this is coming from a retro grouch. The techno wienies are probably thinking computer chips.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.