What changes will occur when the newest generation of Millennial's become our leaders? How will the branches look in 10-15 years?

Not much will change if you think about this most of the Leaders today were only 2 or 3 when Vietnam ended and we are doing great things now. The future leaders will rise and do great things just as we did so no worries here.

I'm an X-er. I remember when the narrative was that we were all self-absorbed losers with no work ethic...much like Millenials. It ended up being wrong. Every generation bitches about the next one and right now, the Boomers seem to be pumping out enormous amounts of hate on their own kids (Millenials are mostly children of Boomers).

I suspect Millenials are shaping up much the same way. They're different but still awesome. They may have less tolerance for things as they are now, but let's be honest, if most of us joined our fathers' Army, we'd think it was outrageously controlling and unreasonable.

Hell, before the GWOT, everyone cried about how the current generation of soldier wasn't prepared and couldn't perform in a real combat environment. They were wrong. EXACTLY the same thing was said about the ODS generation. They were wrong.

I have been around to see a few generations of service members - Vietnam era draftees through OIF. My primary experience has been with Army but I have had the privilege of working with USMC and USAF personnel over my career
While many youth were not shaped by the events and response to 9/11, in my experience the majority of soldiers came from the heartland of America and tended to have values less shaped by national media than people from the larger cities and suburbs.
Our current policy is not primarily influenced by 9/11 - early stages of OEF and the debate about OIF certainly were. Our national strategy and national defense strategy are based upon near to mid term perceived threats in terms of their potential impact on our national objectives.
The administration and following administrations will remember 9/11 or read about it in history books. The question being posed here was also posed in times past by the veterans of WWII and Korea and Vietnam. Desert Storm helped remind the USA that each generation of Soldiers, Sailors, Marines, Airmen, etc. will do their best when called upon. When military leadership is lacking there will be problems; however, the young men and women who make up the next generation that fights will do their best.