To expand on the last point a bit: New modes of transport don't just integrate with the current transport network. Look at bikes - literally hundreds of millions if not billions of daily users worldwide for a century, and yet we're only just now getting wise to the idea that some separate infrastructure for them would be a good thing.
Cars co-opted and displaced carriages from their infrastructure pretty thoroughly. If something compellingly better than walking comes along it could co-opt our current sidewalks. Main reason I don't see that happening is that in order to replace walking you'd have to do it for virtually everyone. That ain't happening.