He didn't just point out a problem, he actively tells people to "stop using JSON" without providing an alternative.

You're right, but that's so easy to do unless the problem matters and is something that people are not easily able to realize on their own.

In this case, I'd say JSON not being the fastest it could possibly be doesn't matter too much. As a result, pointing that out isn't particularly valuable, but could be a bit more valuable if there was an easy-to-adopt solution also being proposed.

But when he said "Stop using JSON" he needs to provide a better alternative, otherwise what is the point of the complain? If he don't have a better alternative, so he should rename to "JSON sucks" or something like that.

If you're going to post against such a widely used format, you had better have a very solid reason, and an even more solid alternative.