How often do you go to places that don't have public WiFi/isn't in your keychain? Actuaries know for a fact it's less than 95% of the time. How? Most (note MOST) people do most (note most) of their traveling within 5 miles of home. Are there occasions where that isn't the case? Yup. About 5%. How much of your time is spent out of coverage (LTE/phone)? I bet it's close to the same.
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Or do you really believe public businesses wouldn't be interested in something that draws customers to them?
The other problem I have is that requires letting Google mine my data. WHY do I need another provider when the correct and simpler answer is put the phone app on the iPad?
While I agree with you that it would be technically easy to add full phone functionality and Apple Watch support to a cellular iPad, and that would be useful for many people, the quoted parts of your post caught my eye.
In many cases free wifi is offered in order to do exactly the kind of data mining that you fear Google does. I recently watched a presentation by the CIO of a large shopping centre operator ar an event run by one of the big wireless networking vendors. The amount of information they get about people who use their free wireless is scary!