Wikipedia has some definitive stuff on cellular/wired connections (note that this does not necessarily mean a decrease in WiFi access):
I’ve just started to try to make sense of it. It doesn’t answer all the questions we have, but it’s indicative of the trend towards cellular.
This is also highly relevant (Akamai’s state of the Internet):
There is also relevant stuff in the web index report.
And the source the Academic community is using is the World Internet Project.
That should give us a fairly good picture and a well researched trajectory of where networks are going.