That would have the effect that every use of str.format for everyone would start producing partially-formatted strings if an argument is accidentally omitted instead of raising an error. Some people might not like that.
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If that is all there is for a downside, this is actually quite weak. You just changed my mind to +1 on the proposal.
Worst case scenario, one goes from one non-running program to a running program producing partially incorrect output. Any legacy code that was not working in the first place, is obviously, clearly, not critical for anyone, otherwise it would have been fixed already.
We can't stal all augmenting to language functionalities because "some is used with the fact writing incorrect code in this way used to produce an error before". Ultimately, by this logic, it would be impossible to add even any new keyword only parameters to any stdlib call, because "there might be some code out there using this parameter, and that used to raise an error"