AWS backup is a bit more nuanced than that; ideally the thing you want is N days of PITR (point in time recovery) and you want that across all your data stores (RDS dbs, buckets, dynamodb tables, etc etc), and you want to be able to restore them all to a common point in time. 7 or 30 days or PITR are common choices. It is ideal if you can perform a data restore in 1 operation since your hair may be on fire when you need to use it. In practice almost all your recovery will be from this.
The storage needed for this depends on the data change rate in your application, more or less it works like a WAL in a DB. What is annoying is that you can't really control it (for obvious reasons), and less forgivably, AWS backup is super opaque about how much is actually being used by what.
Retention of dailies / weeklies / monthlies is a different (usually compliance) concern (NOT operational, not really, if you have to restore from a monthly your business is probably already done for) and in an enterprise context you are generally prevented from using deltas for these due to enterprise policy or regulation (yeah I know it sounds crazy, reqs are getting really specific these days).
People on AWS don't generally care that they're locked in to AWS services (else.. they wouldn't be on AWS), and while cost is often a factor it is usually not the primary concern (else.. they would not be on AWS). What often IS a primary concern is knowing that their backup solution is covered under the enterprise tier AWS support they are already paying an absolute buttload for.
Also stuff like Vault lock "compliance mode" & "automated restore testing" are helpful in box-ticking scenarios.
Plakar looks awesome but I'm not sure AWS Backup customers are the right market to go for.