It needs another separate server which has several disadvantages.
As said various times in this and other threads: You can install it on a Proxmox VE too, documented here:
https://pbs.proxmox.com/docs/installation.html#install-proxmox-backup-server-on-proxmox-ve
Why not extend the existing replication mechanismen to be able to retain old versions based on a plan like znapzend? This would combine backup with availablility
Because this is a much more general approach, independent of any storage snapshot technology. It gives our users the freedom to use the filesystem/storage-technology they want and suits their need best.
If you want availability with whole hosts failing, use ceph. Having backups separated from the live data is a feature.
1) Have a perfect backup history of VMs without wasting to much extra space (compared to extra server or vzdump).
PBS is fully deduplicated, backups are sent incremental and compressed - this saves a lot of space. Having an explicit server for backups is normally a good strategy, as said this is seen as a feature.
2) Revert the VM to an snaphost in case something went wrong without ANY "restore" operation
If restore can go wrong so can rollback, so not sure how that argument would work. Restores should be frequently tested in any backup system, having the data back'd up is only one part of the story.
3) In case of failure of one proxmox server, it would be possible to assign the master role to any other replication target (like now in replication).
You can still use replication if you use Proxmox Backup Server, so this still works.
As any proxmox server can be a backup server and able to tune VMs and containers, there is no single point of failure concerning a backup server and more important NO expensive "restore" operation. Also, the server roles are can be switched at any time, where a dedicated backup server is a special thing.
Yeah, and if the cluster fails you have nothing in your hands, cannot get critical VMs/CTs up on another cluster, ...
You can efficiently sync Proxmox Backup Servers to remotes automatically, avoiding single points of failure.
And as said, any Proxmox VE can also be a Proxmox Backup Server, if you really want. I'd still suggest doing a separate or even off-site remote which the PVE nodes use only a user with limited Permissions, so that if you're PVE host would be taken over they cannot simply delete all backups/snapshots/...
Snapshots are not a backup strategy on their own, they can be worked into that somehow but it could be never as universal and feature full as the approach we went for Proxmox Backup Server.
All in all, you sound like you just want to continue using the replication, it provides already the things you ask us to turn the Proxmox Backup Server into, so...