Orin Hargraves has a good post in the Language Lounge section of Visual Thesaurus on the decay of that good old modal shall, using Fowler’s entry on it as a jumping-off point (“There is never a reason not to consult Fowler about usage: whether you find what you were looking for or not, you’ll walk away from his text amused and edified in a way that you weren’t when you went to it”). Hargraves points out that “Though very few speakers today use it in a prescribed way, shall leaves an indelible impression on the minds of developing native speakers of English in many forms, starting early in life,” citing nursery rhymes, prayers and hymns, and historical documents like the Constitution and the Gettysburg Address. Here is the end of his post:
The upshot today is that shall — besides its fixed modal use in soliciting input for actions — has a special status and is used, not altogether consistently, to impart an air of authority, formality, of loftiness that will would lack in the same context. This usage isn’t lost on Hollywood, which sprinkles dialog with shalls for effect — an effect that Fowler calls “decorative and prophetic” in his original article. Thus,
Greta Garbo in “Grand Hotel”:
I shall dance and you’ll be with me and then — listen — After that you will come with me to Lake Como, I have a villa there. The sun will be shining. I will take a vacation — six weeks — eight weeks. We’ll be happy and lazy. And then you will go with me to South America — oh!
Joan Crawford in “Mildred Pierce”:
I shall prevent this marriage in any way that I can.
You can find scores of other examples with the search “you shall” on script-o-rama.com, where it is obvious that Hollywood is the true master of the “decorative and prophetic” shall. Alternatively, there’s a pretty good case to be made that what Fowler said in 1926 still holds true today: “there are people to whom the English distinctions mean nothing than the discovery that shall & will, should & would, are sometimes regarded as good raw material for elegant variation.”
If you have trouble parsing that final quote from Fowler, good for you; it’s been wrongly truncated and is unintelligible as it stands. For an explanation of what went wrong, see my comment on Hargraves’s post.