Good write-up, Stanc, thanks!
Military procurement has a long and distinguished history of being unable to think outside the box. That's why in those rare cases when it does happen, it's so notable. The current carbine competitions seem to be more of the same -- from what I've read, the pre-determined result is that the Army wants a piston-driven AR chambered in 5.56. At least they're calling for caliber interchangeability at the unit armorer level.
What would be truly interesting is if somebody decided to design a cartridge that out-performs all of the current intermediates (perhaps a 6.5mm-7mm just a little bit longer than would fit in an AR-15), and then designed a rifle around it with a bolt diameter, ejection port and magwell of sufficient dimensions to permit lots of experimentation.