To understand and protect our home planet,
To explore the universe and search for life,
To inspire the next generation of explorers
. . . as only NASA can
NASA's mission is not to develop aircraft and spacecraft technologies in general. The private sector can do that better and cheaper. If it becomes economical, the private sector will do just that. NASA's job is to go places no one has been before, learn things that no one knew, and to share that knowledge with the world. It gives the motivation for the private sector to eventually develop world-changing technologies (like communication and navigation satellites). For this reason, I don't think it's NASA's job to optimize any technology (unless absolutely needed to accomplish its mission).
It would be great to have a spacecraft that could launch from solid rockets on Earth, be accelerated by a scram-jet, be further sent into orbit with liquid fueled rockets, be able to go to high earth orbit, land on the Moon, and come back again with the ability to be flown the next day. This would be an amazing spacecraft. But in the time and money that NASA could spend on making this spacecraft, they could have bases on the Moon, and landings on Mars with much simpler spacecraft while the private sector decided on how they wanted to make a reusable spacecraft.
I should note that my views are somewhat biased due to my libertarian philosophy. Technically, I think that NASA should be owned and operated by various universities in the country (private, of course!). I especially don't want NASA to become a technology developer. That reeks of socialism to me.
NASA does not and never has developed anything. Their job is to propose a mission, provide the funding, and manage the private contractors who actually generate the bulk of the research. It is private enterprise at work; the public funding subsidizes the raw reseach, and it is awarded on a competitive basis. No private company can afford to expose themselves as NASA does; NASA shield private enterprise and allows its technology to advance in quantum leaps. Of course that only works as long as there is a challenge. That excludes going back to a Gemini program no matter how efficient it may be.