> SpaceX had to invent the alloy before the first Raptor full duration fire. That's a very low level, never done before, breakthrough required just to get to the real hard stuff.

Russians had oxydizer-rich gas generators in large engines since e.g. early 1960-s (see Proton 1st stage engines). Oxygen-rich gas generators are at least since mid-1980-s (RD-170). So not exactly never done before.