"Plus, data consumers can deduce much more information from `operator:wikidata` than from `operator`. If, for example, I want a renderer that displays the reporting marks of railroads, I can determine that by checking the statements in the Wikidata item attached to it."

Sorry, but all of those "good reasons" I've heard so far sound like maximum effort for minimal benefit. If I wanted to have something like that within my map/tool, I would write a one-time wikidata query which extracts a table with operators and their reporting marks, and use this to match the normal "operator=*" values.
Something like https://w.wiki/6Rg8
... I wrote this using Chatgpt, all I had to to was to replace the properties with the correct ones - took less than 5 minutes. Another 5 minutes, and it would probably also include all aliases.

Don't get me wrong: I, too, feel it is very important that these "operator" tags are consistent. "CSX Transportation" has been used almost not at all so far, but ok.
But I'm pretty sure that these Wikidata tags -especially on train tracks- cause much more maintaining effort than they would really ever help anyone.