I have an iPhone and there are some excellent Nav programs out there, do a search in App store.
All of the good ones you'll have to pay for, don't know of a good free one. Biggest issue would be mounting it so you could read it (top of tank bag would work, but not great viewing angle).
Other issue would be battery life, running a big App like Google Earth or any of the Nav programs uses up battery quickly.
My vote go for a dedicated GPS, there are a lot around for under $100.00, hardwire it to 12volts and stick it some type of waterproof case.
TBH biggest issue I suspect would be downloading the google earth data. even if you had a 4g connection it will still choke trying to download that amount of data in a reasonable time. I think a mobile map application may work as a hybrid... ie the nmap data is on the phone already and the mobile
data is more of an overlay.. eg wheres the nearest whatever?
battery life on the bike should not be a problem, should be easy enough to get a 12v cable.
the problem you report is common to all small screen devices, and tbh all electronic charting. as you increase the scale you reduce the amount of information available. Im pretty certain you will find the same with Google Earth, just that because you use it on a high resolution screen you don't appreciate what is going on. its certainly true of marine chartplotters, although bigger screens are better to look at but drain power
bear in mind that most mobile phones are VGA resolution at best and modern computer displays are going to be any where upto 3 to 5 times the resolution.. bear in mind its not just the resolutiuon its also the physical size that matters here
Right now I'd agree theres some pretty god mapping GPS's out there. nones perfect, but they are a heck of a lot better than they were 4 or 5 years ago