And if it's being used, and isn't considered problematic, then it should remain. I've found use for all the current ones: (1) for text that naturally has curlies, (2) for templating (3) for immediate interpolation, and improved at-site readability

I see (4) being about the flexibility of (2) and readability of (3). Maybe it'll eventually grow to dominate one or both, but it's also fine if it doesn't. I don't see (1) going away at all since the curly collision still exists in (4).