As soon as you are making a schema for JSON you've eliminated XML with no good justification. More and more, people are re-inventing XML on top of JSON: schemas, namespace, the works.
JSON is great because when you don't need all the things that XML solves it's extremely succint and readable. Use any single copied feature from XML and it turns out being more verbose and less readable than XML.
> People moan about XML readability somehow forget that it uses approximately the same syntax as HTML
No, they don't.
> Imagine HTML in JSON.
Yes, sure, JSON is a crappy text markup language, and would be much less readable than HTML for that purpose.
OTOH, readability when used as a markup language for content consisting largely of prose text and readability when used as a structured serialization format for data that doesn't mostly consist of large blocks of annotated prose isn't necessarily the same thing.
> The point is, XML is quite good for unstructured, semi-structured and strongly-structured data
That's a highly-subjective and controversial point.
(To me, XML is the Java of data languages -- its a lot worse than the best alternative considered on its own for almost any purpose -- though the best alternative will vary by purpose -- but it has a fairly wide range of uses for which its not intolerably bad, and its often a better choice than its inherent features would suggest because of the strength and maturity of the ecosystem around it.)
For 100% of the XML feature-set I don't actually know of a viable alternative. If you are using XML for the right reasons and the right way (rare) there is currently little or nothing that can replace. That being said, there are a vanishingly small amount of problems that actually require XML - namespaces and extensibility are two of them.
> For 100% of the XML feature-set I don't actually know of a viable alternative.
Real problems rarely need 100% of the XML feature set to solve. The breadth of the feature set is why there are lots of problems for which XML is a tolerable solution based on its inherent features (which in turn is a big factor in why it has such a large ecosystem), but they often don't make it the best solution (especially before considering the ecosystem, which is important in choosing a tool, but not a reason to avoid developing a new alternative, since any new alternative is going to start with an ecosystem disadvantage, but with adequate inherent value should be able over time to gather an ecosystem of tools.)
Use the right tool for the job.