There is one BIG difference.
The Android frameworks are already the default on a major platform.
Googles Material Design already looks native on both Android (majority phone platform) and ChromeOS.
Java did not have a major platform where it was the default framework. It was not the default on Windows or Mac, not even on Unices (with the exception maybe of SunOS or Solaris, but not even there).
If Java framework was not the default anywhere, then the primary reason to use Java was because it works everywhere. The primary reason to use the Android frameworks and Material Design is to publish on Android. Having it work everywhere else is a great bonus.
Android and Material Design will not be "write once, mediocre everywhere". It may become "write once, great on Android (majority of phones) and ChromeOS, mediocre elsewhere." But writing for Android does not exclude creating native versions for other platforms. Using Java did exclude creating native versions because that was the reason to use Java, to not have to write native versions.
Android and Material Design will not be "write once, mediocre everywhere". It may become "write once, great on Android (majority of phones) and ChromeOS, mediocre elsewhere." But writing for Android does not exclude creating native versions for other platforms. Using Java did exclude creating native versions because that was the reason to use Java, to not have to write native versions.