[Isofarro]

"And that's the issue here—forcing non-compliant agents to change in order to do what they've always done runs in the face of the Robustness Principle."

You nailed it. Hashbang URLs are not backwards compatible with the existing World Wide Web. And existing tools that use the Web today, cannot in their current state use these hashbang frameworked sites in the same way.

As another commenter noted, hashbangs silo a site within a non-standard requirement. That approach explains why posts such as these overlook or ignore fundamental behaviour of the Web and how it brings together fragments of distributed conversation.

Hashbang URLs break the World Wide Web stack at the HTTP and URL levels, and attempt to fix the damage at the behaviour level. The fix is inferior, and results in breakage of the World Wide Web model (like framesets).

It's a technical implementation of a walled-garden. Walled off from the exisiting World Wide Web.