Regarding BTRFS being 64 bit, that is quite bad actually. I dont have the time to redo the calculations again now, but this is reasoning from ZFS developers. They reasoned something like this:
Today, we are storing Petabytes of data. CERN are storing petabytes, Facebook, Google, etc. To store PB of data, requires something like 2^62 bits. In a couple of years, all data will be doubled, thus requiring 2^63 bits. And in another few years, it will require 2^64 bits. After that, 64 bit filesystems will not do. Then you need filesystems that can store more data than 64 bits. Maybe ZFS would be a 72bit filesystem? And in a few years, ZFS would need to be increased again. Instead, ZFS developers said, "well, let us make 128 bits, and then we never have to worry anymore". Thus, ZFS is 128 bit and can handle 2^128 bits. 128 bit is enough and will never need to be increased. To fill up 2^128 bit, you need something like more atoms in the whole earth or something similar. If you stapled the entire earth with 4TB disks, 10 meters high everywhere on land and on sea - that would be something like 2^100 bits of storage or something similar. You would need several earths like that, to reach 2^128 bits. Thus, 128 bit filesystems is all that humankind will ever need. Laws of physics say so.