How to Prevent File Vault from Encrypting My Drive

There’s no way provided to stop that encryption process, short of rolling in your last backup prior to the commencement of the encryption.

As for whether this was a mistake, I do understand your comment. But you’ve also made it more difficult for the salvage and the hardware-recycling folks and for the hardware-repair-and-replacement folks to access this data. All old hardware eventually gets tossed out, and residual data can’t always be overwritten.

I’d look an an SSD if you’re still running with a hard disk, too.

File Vault is the data encryption. If you turn off File Vault, you’ll go through the whole process here again, this time decrypting all of the data. And the resulting data won’t be encrypted.

Your Mac will be decommissioned eventually, and inevitably. Or your hard disk will fail. Eventually and inevitably, all storage devices will fail. Which means your data will then be leaving your possession. And you may not be able to wipe everything. Whether because the device has failed, or because the data is left stored in bad blocks that have been mapped out, or because the computer was lost or stolen.