> It would be nice if Apple detected a bad WiFi connection and fell back to cell, but it seems they haven't figured out that trick yet.

Apple introduced this very feature in iOS 9 https://support.apple.com/en-us/HT205296

I've never really had time or interest to investigate how well it's implemented, admittedly. It caused some consternation among those with crap data plans at the time, who where understandably upset when the iOS 9 update enabled the feature and lead to unexpected bills.