It sounds fine from a theoretical standpoint to use a pure byte stream rather than reliable messaging, which is what most applications actually want. However, messaging protocols can run byte streams at 0 overhead (you still need a message sequence number/tag, and this field turns into the same thing as the TCP seq num for a byte stream), while byte streams need some overhead to turn into messaging protocols, since there is otherwise no protocol-level delimiter between messages.

This seems like nearly nothing, but when you think about the lost compute power and network bandwidth throughout the history of TCP to this distinction, it's actually a very significant chunk of power, CO2, and wasted human life. Not to mention the lost human life that comes from HOL blocking.

The byte stream abstraction idea, I think, was a suspect decision at the time (born out of a desire to look exactly like a serial teletype port despite the fact that it's the wrong abstraction layer to use for that), and clearly wrong for a transport layer today. Almost everything built on top of TCP re-introduces messaging semantics.

Also, IP never had a message checksum, only a header checksum. The checksumming is at the TCP layer and it is still alive.