I think Gruber was postulating that Apple would continue updating the app to take advantage of iOS in deeper ways while Google wouldn't put the same level of effort into the iOS version of the app, making it stagnate. So more of degrading of the iOS experience than anything related to map data.
He wrote "Google’s maps are going to start degrading", not that their app would degrade. He seems to imply that with less iOS users, they'd get less corrections - which ignores the large userbase of Google Maps on other platforms (Android, web).
The ability to take passive GPS readings and integrate with other parts of iOS is pretty powerful. Imagine if you had a reminder to "Buy new guitar strings" -- your phone could beep and say "You're just around the corner from a musical instrument store -- would you like to get those strings?" Of course, Apple could also decide whether or not to charge the instrument store for this lead generation.
