"The Osprey is not, nor was it ever, a helo replacement"
Herk
For the USAF or the USMC? If the later, I might have to disagree. The -46 is long in the tooth. Been a number of years since I've been on one, but the Vietnam bullet hole patches were plentiful. If you're talking about a leap in capabilities far beyond a simple one for one replacement with a newer generation helo, then yes.
One thing you didn't mention in the debate over the lead service was the number of times the program was almost killed by the DoD. Then SecDef Cheney tried on many occasions to end it all. Finally after some open warfare within congress quietly led by Marines operatives Cheney relented. To paraphrase him, "OK, just don't shove them down my throat" The Marines enjoy a tremendous reputation within the beltway, I don't know if the USAF would have the political clout to pull off the mission given the tenuous situation it was having with procurement.
One could also argue, the Marines have as much experience as any of the services in introducing vertical lift in to the inventory.