No, I don't 🙂
XML it's super slow.
JSON it's only 10% slower than AMF but still slower.
For who uses a Flex client, AMF makes more sense then JSON.
For who whant to share the same web service with thirt parties, could easy reasue the same web services for AMF (for Flex client) and also for JSON.
.NET Core is a very young platform (comparing with the big brother .NET Framework) and implements that was not an easy task and required time, but it's done.
For a few web services to share with thirt parties I agree with you and in fact I do that.
For for a serious application (I mean, a application to relies on web services, I don't). When I mean serious, I mean communication all the time with hundreds of web services. I did the test and the performance difference between AMF and JSON (or worst XML) it's huge (I can't take serious for XML).
When you talk about "nowadays", you forget about half the planet 🙂
Debugging it's not a tedious, it's a plus with tools like Charles.