"Scale up" is fine once you have a working design and can just pup out copies of that design by the dozen with mass production - what SpaceX is doing with Rapto

"Scale up" is fine once you have a working design and can just pup out copies of that design by the dozen with mass production - what SpaceX is doing with Raptors (and did with Merlins) for example. You need the demand to make that worthwhile though.