Re: Not sure about this
"If you go for new protocol at the transport layer that introduces issues issues of its own."
Pssst... my memory's not what it used to be, but my recollection is that the stuff you mention was all doable (and documented and available and testable) in the 1980s in a standards based and vendor independent way, but the processing power (silicon *and* mental) was more than people were generally prepared to pay for. Now the silicon is cheap. But the necessary software engineering principles are nowhere to be seen.
Back then, part of it came from DECnet, part of it came from OSI higher layers. Maybe some other stuff too - after all, the real point of this stuff is what the data *means*, not how the bits behave as they get from one box to another or one app to another (and in a few years time we'll likely be back in the dark days of one set of proprietary apps talking to the same vendors apps on a different box, with no useful interoperability).