Indirect object is similar to dative, sort of.

Seriously, most people would say "Who does this belong to?" We can argue whether it is good style or not, but descriptively it is standard grammar.

Googling "to whom it may concern" returns ~2050000 results, "to who it may concern" returns four time as less results (~479000).

You are not right.

> Googling "to whom it may concern" returns ~2050000 results, "to who it may concern" returns four time as less results (~479000).

"Four times as less" -> "four times fewer" or "one-fourth" ??

"To whom it may concern" is formulaic though, just like "Dear Mr. President."

Those are words used as they are, verbatem, in a specific context. They are thus ossified and preserved in a much more conservative state than the rest of the language.

It is not unreasonable to suppose that even as our language changes, that phrase will continue to remain intact in root and morphology even if the morphology disappears elsewhere.