> been outside of low earth orbit

we stopped pushing that frontier because we've been there and found it dull and empty, as a matter of fact, we decided zero g experiment were more important than playing with regolith, and build a freaking human habitat in the void to support that endeavor. beside, we've got regolith here on earth now, we can experiment on it in the commodity of our backyard if needed be.

> mach 3 flight

except the routine space launches that ship scientist back and forth from the space station, going through literal plasma on the descent leg.

> Roman concrete

this has just been widely romanticized, we don't use that because it's incompatible with fast building processes requirements, is too heavy for large structures and it's hard to dismantle when a structure is no longer needed or need renovation.

sure, some other stuff one can cite as having reached an apparent sub-optimal minima, like average speed of travel, but that's not because we're backward, it's because we're learning to take care of our own environment. slowly, but we're getting there. just because the tradeoff are different it doesn't mean that a golf id3 is just worse than a bugatti eb110 or a recess from peak technology.