> Isn't the average Japanese taxi some big Toyota Crown sedan already? How much more premium than that can you go?

Toyota Century :)

https://www.wsj.com/articles/japans-new-emperor-picks-his-ri...

Edit: Super short TL;DR from the article

> Introduced in 1967, the Century isn’t for the flashy or hasty. Prospective customers can wait months because Toyota hand-builds each one. Background checks ensure the buyer is of an appropriate caliber, and not a member of an “antisocial organization”—a Japanese euphemism for the yakuza crime syndicates.

It's only sold in Japan.

Edit 2: Some more information courtesy of Wikipedia - https://en.wikipedia.org/wiki/Toyota_Century