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It is good to have more than one designSpaceX has multiple designs generations ahead of anything in Europe in this category. (China is catching up, but it too is retracing Hawthorne's path.)
Themis is cool. But it's duplicating what SpaceX did fifteen to twenty years ago and what China has been working on for ten. Methalox, open cycle gas generator, steel tanks...there aren't any daring design decisions here. And there shouldn't be. This is a solved problem. (Again, that doesn't mean Europe shouldn't be solving it independently. But it's not innovating anything here.)
Consider an alternate universe where NATO isn't in shambles. Europe can use SpaceX for LEO. And in the meantime, it can focus on other designs, other categories. Maybe a different fuel. Maybe a novel engine. Maybe something entirely extra-atmospheric. Maybe it's a breakthrough in satellite design, or habitat science. Instead of replicating Falcon 1/9 and Raptor, there could be a genuinely new design.
We don't get that, because Europe has to secure its launch sovereignty. That is the cost.
> We could maximise economies of scale by having one car manufacturer that made a small number of models
Mature market. Multiple optima. See my comment on Airbus and Boeing. When you're pathfinding, you want multiple bets. When you're pursuing, R&D benefits from scale.