You enabled a feature where low-res photos will be stored locally on your Mac, but the original resolution photos will be stored in iCloud. So when you are disabling sync, iCloud choses to delete the low-res photos on your Mac, and keep it in iCloud.

Method 1: Contact Apple support, or walk into an Apple store and ask them for help.

Method 2: Try finding and disabling the feature I mentioned in the first paragraph. If it asks about deleting photos while disabling this option then don't use this method. After you disable that feature, then disable iCloud backup

Method 3: You should try downloading all the photos currently stored in iCloud and put them in the same (create a new folder) folder as the ones that have not been uploaded(make sure they are original resolution)(as a back up). Make sure there are no duplicates. The number of photos should add up to the total number of photos you had(2,073 in your case). Then just disable iCloud photo library. Then just delete all the photos in your photo library, and put the all the pictures that were in the folder you created earlier into your photo library.

You enabled a feature called "Optimize Mac Storage" where low-res photos will be stored locally on your Mac, but the original resolution photos will be stored in iCloud. So when you are disabling sync, iCloud choses to delete the low-res photos on your Mac, and keep them in iCloud.

Method 1: Contact Apple support, or walk into an Apple store and ask them for help.

Method 2: Try finding and disabling the feature I mentioned in the first paragraph. The alternative is called "Download and Keep Originals. This will only work if you have enough space on your device. After you disable that feature, disable iCloud Photo Library

Method 3: You should try downloading all the photos currently stored in iCloud and put them in the same (create a new folder) folder as the ones that have not been uploaded  (make sure they are original resolution, as a back up). Make sure there are no duplicates. The number of photos should add up to the total number of photos you had(2,073 in your case). Then just disable iCloud Photo Library. Then just delete all the photos in your Photo Library, and put all the pictures that in the backup you created earlier into your photo library.

You enabled a feature where low-res photos will be stored locally on your Mac, but the original resolution photos will be stored in iCloud. So when you are disabling sync, iCloud choses to delete the low-res photos on your Mac, and keep it in iCloud.

You enabled a feature where low-res photos will be stored locally on your Mac, but the original resolution photos will be stored in iCloud. So when you are disabling sync, iCloud choses to delete the low-res photos on your Mac, and keep it in iCloud.

Method 1: Contact Apple support, or walk into an Apple store and ask them for help.

Method 2: Try finding and disabling the feature I mentioned in the first paragraph. If it asks about deleting photos while disabling this option then don't use this method. After you disable that feature, then disable iCloud backup

Method 3: You should try downloading all the photos currently stored in iCloud and put them in the same (create a new folder) folder as the ones that have not been uploaded(make sure they are original resolution)(as a back up). Make sure there are no duplicates. The number of photos should add up to the total number of photos you had(2,073 in your case). Then just disable iCloud photo library. Then just delete all the photos in your photo library, and put the all the pictures that were in the folder you created earlier into your photo library.