I heard someone say this morning, “Alls I want is a standing order.” This misspeak is spreading like grass fire. I began hearing it in the 80s, too, but the instances were few and far between. It was mostly the locals on staff but I heard a few students say it, too. I quickly corrected them in a threatening voice.
We can’t be sure if it is alls (plural) or all’s (contraction). I can’t think of a grammatical function word that ends on an S that is so frequently used with all that it would form a contraction with it. All that and all are are frequently heard together, but a contraction with these words would produce all’t and all’re, respectively.
So we are forced back to my first hypothese—unless there is a third one. All are is a plural construction but most words occurring before are end on an S. So, for consistency, an S is added to all.
Do you have a better idea?