I rewrote some parts for scalability in the first couple years after the acquisition, and a lot was changed after I left. But I believe a lot of it is still there 15 years later. Perhaps a current maintainer could elaborate?

Many codebases end up pretty different after 15 years. It doesn't mean the original codebase wasn't a useful starting point. It's lower risk to adapt and improve based on customer feedback than figure out, "what should a build-your-own e-commerce tool do?" in the first place.

Paying users. That's something very valuable. The fact that is still running says a lot about the product.

That's exactly what they bought. These days you can clone a lot of sites that exist out there (HN or reddit are great examples) but you wont get anywhere without the community