I often like to come up with new* words, like "unrememberable", and when people call me on it because it's "not a word", respond by saying "it's a valid combination of morphemes". Which is absolutely true. They also can't formulate a response, because they've never actually taken a course on linguistics and don't know what the fuck I"m talking about. Grammar Nazi's know nothing about language. We have sort of an obsession in the modern age with "correct" grammar, treating it as something objective that should never change. But human language isn't a formal system like math. There's not anything objective about it. It's honestly damaging to language to attempt to pretend like it's a formal system.
And really, some pieces of "incorrect grammar" actually make the language more clear. For instance, English used to have a singular second person pronoun, thou, and a plural form, ye. Then that fell out of usage, and was replaced with you. That leaves English as one of the only languages without singular and plural second person pronouns, and is inconsistent with the first and third person pronouns of the language. So peoples develop "y'all" and other variations of the second person plural in order to make the language clearer, more concise, and more consistent. But then grammar Nazi's come in and declare that "wrong", as if some child had answered an arithmetic question in the wrong way. How on Earth does that make sense?
At best, this could be said to be a matter of aesthetics. Some people like their language tidier than others do. Some people dislike Justin Bieber, others do not. It's not a case of one person being "wrong" and the other being "right".
*BTW, I spelled this as "knew" when I first wrote it out.