I don't really understand why they would release something "worse" than Opus 4.6. If it's comparable, then what is the reason to even use Opus 4.6? Sure, it's cheaper, but if so, then just make Opus 4.6 cheaper?

It's different. Download an English book from Project Gutenberg and have Claude-code change its style. Try both models and you'll see how significant the differences are.

(Sonnet is far, far better at this kind of task than Opus is, in my experience.)