I know why it is; I know that language changes, and I know that one task of lexicographers is to record those changes. But I don't think they should give the same status to a usage that is just a decade old, and restricted to casual chit-chat among teenagers, as one would accord to a usage that is established over centuries.
According to TFA, "nonplussed" is from Latin "non plus" (no more), and it doesn't seem to have ever had that meaning in English. So I don't really care about "nonplussed". I do care about "literally" (by the letter), and the fact that lexicographers treat its usage to mean "figuratively" as perfectly legitimate. At least, the dictionaries should point out that the version I consider wrong is slang.
The result is that the word "literally" can't now be used in precise discourse, and you have to find some awkward circumlocution. This kind of abuse makes the language less expressive, and is cause for regret.
I had to check m-w's entry for "literally". While they do include a sense 2 that is similar to "figuratively", there is a substantial note afterwards and an FAQ that does a great job of explaining the status of the two senses and a touch of their history. They give some facts that contradict your above, as well (related to the timeline, as well as what constitutes slang). In case you're interested: https://www.merriam-webster.com/dictionary/literally
I do think this is what the dictionary should say. It describes actual usage, including disagreements about it.
By the way, I do think there is a place for linguistic prescriptions, it's just in style guides rather than dictionaries. E.g., fine for the Economist to decide "literally" will only be used in its literal sense in their pages.
According to TFA, "nonplussed" is from Latin "non plus" (no more), and it doesn't seem to have ever had that meaning in English. So I don't really care about "nonplussed". I do care about "literally" (by the letter), and the fact that lexicographers treat its usage to mean "figuratively" as perfectly legitimate. At least, the dictionaries should point out that the version I consider wrong is slang.
The result is that the word "literally" can't now be used in precise discourse, and you have to find some awkward circumlocution. This kind of abuse makes the language less expressive, and is cause for regret.
Why can't the English learn to speak?
~ Prof. Henry Higgins