Source
When this applies
when the 'obviously superior on paper' choice of material/technology is causing unacceptably slow real-world progress
The takeaway
When a nominally superior material/technology requires unbuilt infrastructure or extreme precision to realize its advantage, a cheaper, more workable 'inferior' alternative can outperform it once second-order effects (cost of supporting systems, iteration speed, derived mass savings) are included.
Why it works
Carbon fiber's theoretical mass advantage only materializes with large-scale defect-free curing infrastructure (giant autoclaves) that didn't exist; steel's 50x lower raw cost, easy outdoor welding, and better cryogenic strength-to-weight plus higher heat tolerance (reducing heat-shield mass) made it net lighter and far faster to iterate with.
One level up
Re-evaluate first-principles material/technology properties under your actual operating conditions (e.g., temperature regime) rather than trusting conventional wisdom (room-temperature intuitions).
Where it stops holding
This stops working when the operating conditions actually match the conventional-wisdom assumptions (e.g., room-temperature, static structures), where the theoretically lighter material truly wins.
Detail
situation: Starship was being built from expensive, hard-to-cure carbon fiber; progress was extremely slow, with wrinkle/defect issues requiring an autoclave bigger than any ever built.
decision: Musk personally pushed the switch to 300-series strain-hardened stainless steel, informed by historical Atlas rocket steel balloon tanks.
outcome: At cryogenic temperatures steel matched carbon fiber's strength-to-weight, cost 50x less in raw material, was weldable outdoors, and its higher melting point cut heat-shield mass enough that the steel rocket ended up lighter overall than the carbon fiber version would have been.
lesson: Re-examine material properties under actual (not room-temperature) operating conditions before committing to the 'obviously light' choice.
In their words
So basically almost the entire primary structure is a cryogenic temperature. So then you've got a 300 series stainless that's strain hardened because almost the whole thing is a cryogenic temperature, actually has a similar strength to weight as carbon fiber. It costs 50 times less than raw material and is very easy to work with.