wrote:
No major advances in basic propulsion science, i.e. no dilithium crystals,
impulse drive or anti-gravity engines nor is there likely to be.

Until 2053 :-) (for warp drive, the rest will be transfered technologies from Vulcans)

That's not to say costs can't be reduced, just that it is unrealistic
to expect a couple of orders of magnitude reductions.

I agree entirely.

However, this brings a big dilemma: does one adopt a defeatist attitude and
stop looking for new and improved propulsion methods limiting humanity to
current rocket engines for the foreseable future, or does one continue to
research new propultion methods with failures until they actually come up with
something that works ?

How many rockets exploded/failed in the 50s and 60s until they got it "right
?" (heck, Ariane has had a lot of recent explosions).

The engine isn't the only story. Thermal protection is another big issue.

What seems to be stopping NASA right now is a religious choice between a
reusable vehicle and a simple capsule, and for reusable, whether it should be
winged or not, parachute or not.