> This was a well-known design choice where the user could balance between transaction size and privacy--people who preferred faster/cheaper transactions chose to sacrifice untraceability. Nowadays, the default option enforces higher untraceability.

I don't think you understand the flaw behind this "design choice". While its great to give the user the choice in how they want to balance things, the individual's choice affects the whole system. If you - as kess privacy-conscious user - use 0 mixins (as was possible) or the min. no. of mixins, when your transaction is used as mixin for a privacy-focused user, you make that user much more identifieable. The choice defeats itself when you depend on others to take at least the same number of mixins for their transactions.