7% in two months. What kind of adoption had you expected? Everyone won't switch over night, that's the whole problem behind why flag dates in distributed systems are hard, and why many people adocate backwards compatible changes whenever possible.

> if Core simply agreed to double the block size

Core is the name of the Bitcoin software. Perhaps you mean the Bitcoin community? The functionality needed to roughly double the block size was released last year and finally took effect in August. If that's too slow for your use case, then you are probably on the edge of where Bitcoin is useful anyway. It's hard to imagine any sort non-contentious hard fork planned and executed in less time than that, without endangering other people's money.