Should music streaming services be compelled to support MusicBrainz to support this GDPR case simply because it is commonly used? Who decides that mbid is the GDPR-accepted track identifier?

In contrast to requiring a particular standard to represent metadata, the better way is to require that the exported data be machine-readable and its contents documented. This gives providers the flexibility to innovate, adding their own identifiers or whatever useful data they want, but avoids legislators forcing everyone to use a common standard that, while useful today, can quickly go out of date. Competitors can then write converters or importers based on this documentation, as could motivated FOSS users.