If you need to consider how to buy your drives so you can use a filesystem, that’s a flaw of said filesystem not an edge case.

It clearly is an acceptable one for a lot of people but it does leave space for alternative designs.

This is the way it's always been. RAID can't really handle mismatched drives either and you must consider that when purchasing drives. It's not a flaw, it's a consequence of geometry

It’s strange you say that because my btrfs array handles mismatched sizes just fine.

Let's say you can put a hdd and ssd of different sizes in raid1, but doesnt mean you should.