I'm using BTRFS on many VMs running Ubuntu servers, and I find that expanding the virtual hard-disk, on Proxmox, without stooping the VM it's far trivial.

I think you missed a key word there so the meaning is lost. Is it "far from trivial" or "far more trivial"? You also surely meant stopping, not stooping.

I think RAID 5/6 will be dead before btrfs.

How are you doing redundancy then? Raid-1?

Me personally? Yes, RAID-1.