Thanks for the question! It should be trivial to download the export files from the storage bucket and store them elsewhere. I'm not sure if the API would support an external destination directly (S3, for instance), but my guess is that gs:// URLs are the only supported destinations.
Migration may be a bigger issue. It appears that the files generated by this API are in the same format as Google Cloud Datastore entity exports, which appear to be proprietary. The decision to use Firebase comes with a ton of vendor lock-in, so this is not surprising.
If anyone is looking to export their data for use in another system, one of the tree traversal scripts found on NPM would likely be their best bet. Most of them are capable of producing JSON files that could be consumed by other systems.
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