What do you call 'deep space' ?

The only missions I'd classify as deep space in my lifetime are Galileo (launched on STS), Cassini (Titan IV) and NH ( Atlas V )

To assume that faster is better is to miss-understand the basics of spaceflight. If you're going to a comet and coming home, if you're going to Mars, if you're visiting an asteroid - absolute speed is not important.

You send MRO to mars on a Delta IV heavy, and it's never going to stop because it doesnt have the delta-V to get into orbit.

You send Stardust to Wild 2 on an Atlas V and it's never coming home again.

And lest we forget, that vehicle last night cost approx 3 x the price of a Delta II. If you dictate the retirement of Delta II then you eliminate the discovery program which has given us Pathfinder, NEAR, Genesis, Stardust and so on.

There was also a reason why NH2 went so fast. It's less than 500kg - a tiny fraction of the last two spacecraft to go beyond Mars.

Furthermore,