That's what happens here. This person was a Professional Engineer, a kind of certified engineer. Their job is to determine if stuff is safe for use.
Things like roads, bridges, pressure vessels, and apartment buildings all have a Professional Engineer (PE) look over their designs at some point. Their job is to use their expertise to determine if the thing can do it's job safely.
Existing structures are also periodically inspected by PEs. Like the above person, they determine if the thing can continue to do its job safely.
Unlike other engineers (in the USA), PEs have to attain a certification. You need to take an exam and maintain a license. It takes a lot of time and effort to obtain these things.
Shouldn’t the inspector be working off existing plans rather than being able to declare any structural element as decorative?
As someone else mentioned, the building is old enough that the initial plans likely no longer exist.
It's the job of the PE to make judgment calls like "is this column structural or decorative". This person fucked that up. Losing their license and ability to practice would be a fitting punishment.