Language change is, generally, organic. It usually doesn’t happen by fiat (especially in English); it also doesn’t happen by vote. There may be some influence from “above” by people such as English teachers, but that mostly affects what rules people think they’re breaking when they’re speaking the way they want to speak anyway. You could say that language change – grammar, the meanings and pronunciations of words, and so on – happens by mass popular movement.
Which is not to say that it’s populist. Populism is a political stance that advocates the people – the general populace, hoi polloi – in opposition to a ruling élite. Language change is not the product of a program leading a movement of the populace in opposition to English teachers, editors, and others. There could be such a movement, of course, but de facto language change happens by popular will anyway. We’re all part of it, not just the people at the top. (The situation can at times be different for languages with official deciding bodies, such as in France.)
So, you could reply, if we all decided that the meaning of populism should shift to ‘following the will of the people as with a general tide, without a specific political program’, then that would be the meaning. Well, yes, if that sense shift happened and ultimately overcame opposition. The change would probably take quite a while and not be without some controversy. But it could happen.
But there are shifts that we have every right and reason to resist, no matter how many people use the new sense. We are all streamkeepers of this flowing language. When a word is being used as a euphemism to let something slide that should not, or when its use carries implications that have negative consequences, we should not let these pass unnoticed. If we speak up and point out the problems, we may help these shifts to become unpopular.
So, for instance, for a long time there was a default assumption that people in certain roles were masculine, and so he and him were used. Around the time that such assumptions started to be a bit less tenable, a common line from prescriptivists was that he and him were the natural universal gender-neutral pronouns. (Poor men, having to sacrifice the uniqueness of their pronoun! Ah, such sacrifices must be made.) At long last enough people pointed out that this did, in fact, convey the default assumption of masculinity – words carry resonances and implications whether you say they should or not – and so use of masculine pronouns as a universal has lost general acceptance. (Read more about this in my article on they.)
Which, I suppose, you could say was a populist movement – the neglected masses against the prescriptive authorities – though it was in particular a movement by and on behalf of the more neglected moiety of the populace, to influence the more dominant segment and thereby produce a fairer outcome for all.
But now let’s say that people start using populism to refer to a movement focusing on the desires not of the whole population but on a minority of it who consider the remainder to be of lesser status. Say, for instance, that there is a group we’ll call X in the population, and they feel that the government has been giving too many rights to that larger part of the population that is not X. This group has traditionally been the group that, for all its internal differences, has been ceteris paribus the more-advantaged group, and they’re seeing non-Xes get similar rights. This doesn’t involve the loss of any rights from X – unless you consider it a right to have things that other people you consider inferior can’t have. If some political leader or party rallies members of X against the government just so they can protect their perceived right to have more rights, would you call that populism? Would you accept seeing it called populism? When the movement is for the rights of not all the populace but just a subset of it, and strongly against the rights of others?
This is not a hypothetical question. I’m seeing populist used quite a lot by news media and the commentariat for racist, nativist, frankly sexist and reactionary movements. In countries across Europe and at least one in North America, leaders who advocate or enfranchise not just xenophobia but racism and sexism are being called populists, and the reactionary groups that support them are being described as having populist sentiment.
Which implies that women and non-white people are not part of the populace, or anyway are not relevant parts of it. In spite of being, in sum, the majority. And, for that matter, it also implies that white men are, en masse, in favour of such movements. Which is also not true.
I think we owe ourselves and everyone else a duty to make this use of populist and populism unpopular.