Dicho says: "It is usage which determines correctness or incorrectness".

If that were entirely true, then every single grammatical misusage noted in this thread, the "For pedants only" thread and any other thread that deals with the subject would be correct, since they are all in use. However, they are of course incorrect in the PRESENT state of the language.

It is true that language does evolve. Common grammatical and pronunciational mistakes often gain acceptance with time (in that sense, the language devolves rather than evolves!) but it seems that the people who use the correct versions must die out before the versions themselves do; it took many generations (150 years?) for the definition of "chomp" to change to include "alternate of 'champ' ", and even so "champing at the bit" is still acceptable and perhaps preferred (in your dictionary, Dicho, is "champ" defined as an alternate of "chomp"? If not, I venture to say that the writers still don't consider "chomping at the bit" to be on equal footing with "champing at the bit").

Even when a word or phrase becomes "tolerably respectable", it still isn't necessarily "proper" (appropriate or suitable). For instance, we still teach our children that "isn't" is correct and "ain't" isn't! Correctness certainly determines usage to a great extent; otherwise, how would we have learned to read? We need the correctness of rules and definitions to keep the language from devolving to the point at which no one can make himself understood to anyone else who supposedly speaks the same dialect!