The problem is that free public WiFi is wildly variable in quality, whereas 4G is usually quite good. Most of the time I just tether even if there's WiFi available since I can use my data plan in most countries and the WiFi is usually disappointing.
Also, many countries now require public hotspots to verify the user by getting them to receive an SMS. In some countries it even has to be a local number, because they have compulsory SIM registration, so requiring a local number lets them tie the Internet usage to an identity.
I fear things will go in the opposite direction with ubiquitous 4G. Very often 4G is already much faster than most WiFi hot spots, to the point that I don't even bother anymore and just use personal hot spot on my laptop.
Colombia has lots of free WiFi in parks, for example. We should be doing this everywhere.