A gentleman in a private discussion group has asked how new usages and changes in usage come to be accepted, and who decides. These are questions worth wider discussion.
One need only look at the history of the word nice, which Merriam-Webster.com has recently traced, to see how variously and radically the meaning of words can change over time. And not just words, but grammar as well. The second-person singular thou, thee, thy, thine has essentially dropped from the language except in quotation from the Authorized version, the verses of eighteenth- and nineteenth-century hymns, and affectations of archaism.
One way to understand changes in usage is to see them as fashion. Think 1970s menswear—those neckties as wide as a lobster bib, those lapels like the tail fins on a 1950s Buick. Or think of the ridiculous trilbies hipsters currently affect. Words also come into vogue; some are scorned but display staying power while others fade.
Teenybopper had its day, as did groovy. But contact, widely scorned in the 1940s and 1950s, has proved useful, and the hopefully shibboleth of the 1960s and 1970s is fading away as those who object to it pay Charon their fare. The jury remains out on impactful.
About impactful: It is, on its own terms, innocuous, an adjective formed regularly from a noun, meaning "having a significant or powerful effect." It is not scorned for any of this, but because of the people who use it, which was the same reason for scorn previously heaped on contact and hopefully. Many sticklers, as they go on about preserving the purity of the language, are merely advocating a particular fashion that is to their taste.
English, like other languages, is crowdsourced (a useful word that is only about a decade old). If enough speakers and writers of English use a particular word in a particular sense, or a particular grammatical construction, long enough, it becomes standard. This is how it has always worked.
Spoken English, particularly slang and colloquial English, is typically the developing edge of the language; standard written English, the language of academia, law, journalism, and the other professions, is a conservative brake on the language. When I started work as a newspaper copy editor thirty-six years ago, there was considerable resistance to accepting gay for homosexual. Common usage wore down the resistance.
So the answer to "Who decides?" is "We do." The lexicographers follow behind us trying to puzzle out what we mean.
Do not imagine that Kory Stamper and Emily Brewster at Merriam-Webster and Steve Kleinedler at American Heritage sit on gilded thrones as underlings approach, bow, scrape, and present individual words on velvet cushions for a thumbs-up or thumbs-down. Peter Sokolowski does not play a trumpet fanfare as a new word is admitted into the sacred precincts of the dictionary.
They are instead examining the vast electronic corpora of spoken and written English that have become available in recent years. They seek to identify nuances of meaning. When novelties crop up, they can see who is using them and where, how widely they are in use, how long they last in use. They record and describe; they do not decree.
That leaves the decisions up to you.