Space Launch System's design called for the integration of existing hardware
I would much rather them use existing tried tech and incrementally advance them rather than try a radical new design.
The reason for incremental development is that your engineers and technicians learn their "craft", gradually learn where they can shave off millimetres and where they have to add more. Work out what works better than expected and what is clumsy and stupid and needs to be redesigned. A kind of guided evolution of technology.
However, the first couple of flights of SLS will be using actual Shuttle orbiter engines (SSMEs) salvaged from the three retired orbiters. Experimental, first generation, beyond-the-state-of-the-art-at-the-time, hideously complex and overengineered engines which haven't been in production since the late 1980s and whose designers are all in nursing homes.
Most decidedly not using "proven technology, incrementally advanced."
but if we can tweak existing tech, and make it useful for deep space why not??
SLS and the Orion capsule are costing around $3 billion per year during development. The first manned launch will be no earlier than 2021, and insiders suggest that deadline will slip several years. But from now until 2021, ignoring the tens of billions spent so far, SLS/Orion will cost $21 billion in development before the first crew is launched. However, that configuration is only capable of reaching the moon and back, carrying no cargo besides the Orion capsule, and the capsule will only have 14 days life support. By the time the SLS Block II and Orion's long-duration service module are developed for deep space missions, around 2032 (plus delays), the cost will be over $50 billion (plus overruns). That, of course, doesn't include actual launch costs; nor does it allow for developing any mission hardware, such as landers/rovers/surface-habs/etc.
That $21 billion would buy 140 Falcon Heavy launches, or about 7000 tons of payload. The $50 billion could buy over 300 FH launches, or over 16000 tons of payload. The equivalent of more than two full International Space Stations every year.
Or more realistically, four FH and one F9/Dragon, 200 tonnes and 7 crew, for just $750 million per mission, up to four missions per year for the same budget. Or, starting in, say, 2019 to mark the 50th Anniversary of Apollo 11, you'd have $15 billion free to develop additional boosters/landers/rovers/habitats/etc, then two missions per year, leaving $1.5 billion every year for other projects, hardware, and operations.
In other words, the opportunity cost of SLS/Orion, ie, what they prevent, is enormous.