re: "I can’t see the advantage of greatly increasing the speed of the delivery system..."
Addressing only the millitary question, thinktankers get wet dreams about greatly increasing the speed of delivery systems. We will fight future battles at processes that approach the speed of light. Not getting a handle on hypersonic flight when such a handle is feasible would be handing an advantage to one's potential enemy. At least that's what my inner cold-warrior tells me.