No object lock API would help you if the business where you rent the service is going out of... business. With all your data - and you are without.
Or even fancier - just changing the price for the egress.
Or even fancier - just changing the price for the egress.
The probability of a hard drive failure or ransomware at the same time as backblaze or aws going out of business is pretty much not worth thinking about.
The probability of an attacker using the api key in your backup script to destroy the backups is far more possible.