There have only been 2 reusable launch vehicles ever, and both were only partially reusable. Shuttle and Falcon 9. Falcon 9 has already made massive strides in reducing any refurb costs between missions. there is no reason that you couldn’t build one that requires no maintenance in between flights for the same reason that there’s no reason you can’t make an upper stage that can do multiple firings per flight without any maintenance in between. Upper stage reuse will be trickier, but ultimately there’s no fundamental reason why refurb has to be done in between every flight.
Do they deliberately deorbit the Falcon second stage? Would be interesting if we could keep a couple parked in LEO and refuel them for different missions.
Reusing second stages in space only really makes sense once you are making your fuel in space.
For a lot of purposes, Starship should make it extremely cheap to send propellant from Earth. $10/kg in LEO. Cheaper than any lunar mining proposal I’ve seen.
That said, I hope we do make propellant off-Earth eventually.
I am skeptical of the claim that they will just be able to refuel and go. Every reusable space vehicle has had that goal and always cost more than expected to refurbish for the next launch. Yes, we may get there eventually but not in the next few decades, the risks of a catastrophic component failure are too high to do no refurbishment.
The starship and super heavy are certainly cheaper to construct but that has more to do with fabrication techniques than the material cost.
Methane is less dense than kerosene so needs larger tanks but it does have more energy per unit mass. The overheads of using cryogenic fuel are reduced on a larger rocket but the performance gain is probably only 10% or so.
I am not aware of any significant insulation between tanks on the Falcon 9 and it certainly isn't a major cost factor.