One thing to note of disallowing 64 byte transactions on the network is that this will not work well if we ever decide to move away from a 256 bit hash digest for our merkle tree structure.
For instance if we decide to move to a 512 bit digest, we would now disallow 128 byte transactions, which by my count ~300,000 occurring in the bitcoin blockchain. A 1024 bit digest would disallow 256 byte transactions, of which there are ~450,000 occurring in the bitcoin blockchain.
More generally with this approach it seems if have a N byte digest, we cannot allow N*2 transaction size?
Ig if such a drastic change is needed, maybe we propose just reworking the entire merkle tree structure.