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> Some negatives:
The first and the foremost: we would need two totally new protocols in place of two already well-known, well-functioning and well-tooled ones. Benefits you mentioned are negligible.
If SCTP did become the protocol we transition TCP/UDP to - should port numbers stay? (iyo)
If you want to get rid of port numbers, you will need to roll a new protocol, and nobody will go this way merely because of four bytes per packet.
Also, there would probably be vast amount of unexpected traps and pitholes without port numbers. We already know well how to work with port numbers. We don't know too well how to work without them.