Th iCloud photo library is not a backup. I understand it's what people were expecting but it's simply (and rightfully so) a centralisation of your photo's. Keeping all your devices the same and giving web access is it's purpose and that is very useful.
Backup should never be done in the cloud. You need to manage your own backups.
Having saidf that, Onedrive will backup your photostream into their cloud which will not replicate changes such as deletions from your devices. They will give you 30gb for free. You could use that as backup in the hope that Microsoft don't lose it. But, they also then co-own those pictures.