It's not clear to me that designing something that needs to be assembled successfully once vs twice vs 10 times doesn't introduce some more cost tradeoffs in itself. Just around tolerances and yield even, how many pieces are required to be made to give one complete set of parts that all work together vs 10 complete sets that all work together. I could see some extra pain and assembly from having to get more permutations of part variances all working together and validated. A while back, at least, a lot of the testing processes for space components were VERY manual so you'd either need to invest more for automation or accept linear costs on all the extra production/verification.

This is partly quibbling over trivial details - I would imagine the savings is closer to 50-75% than 90% - but at a certain point those differences in tens-of-percentage points are lots of billions of dollars.

Here's the real kicker to me: why spend even an extra 10% up-front before even deploying one of them? Why not spend the marginal costs on building an additional unit only if the first fails? Then you can learn from the failure too? And if it works the first time, those billions can be spent elsewhere instead.