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Aren't the backups now dependent on (1) the storage provider and (2) the Arq software developer?

You lose Arq, you lose your backups.

The format Arq uses is open and documented, so that you can restore without having Arq if necessary:

And there are tools to help:

The format of the backups is specified, so making a replacement should be possible if needed: https://www.arqbackup.com/s3_data_format.txt