"Then people trying to write formally followed the grammarians"

Similar confusion over split infinitives. The folk lying down the laws on English grammar tried to make it follow the rules of Latin. Which it can't - English is, I suppose, essentially a Germanic language.

Latin infinitives are only one word - eg ire - and can't be split. English infinitives are two words - eg to go.

So, despite what some will try to tell you, there's nothing wrong with "To boldly go".