If you want prescriptivism in harmony with descriptivism, you might want to choose a better example than "will" vs. "shall". To quote Merriam-Webster's Dictionary of English Usage:
In America, of course, "shall" is pretty much not used at all, and makes you sound snobbish rather than formal. But whatever the case, even if the rule had been adhered to at some point in history, the purpose of prescribing it once language has changed eludes me. What is the problem with language evolving? It does it all the time, and no amount of rules will stop it. The only thing these kinds of rules accomplish is making people feel superior and, oftentimes, look silly. It's almost a certainly that any of these rules that people keep bringing up is routinely broken by Shakespeare. He must have been a terrible writer. Here are some examples I dug up:
"No, I will be the pattern of all patience; I will say nothing."
"I will live in thy heart, die in thy lap, and be buried in thyeyes—and moreover, I will go with thee to thy uncle's."
"I will speak daggers to her, but use none"
"ourself will mingle with you and play the huble host"
"I will have such revenges on you both,
That all the world shall--I will do such things,--
What they are, yet I know not: but they shall be
The terrors of the earth. "
Quote
Our conclusion is that the traditional rules about shall and will do not appear to have described real usage of these words very precisely at any time, although there is no question that they do describe the usage of some people some of the time and that they are more applicable in England than elsewhere.
In America, of course, "shall" is pretty much not used at all, and makes you sound snobbish rather than formal. But whatever the case, even if the rule had been adhered to at some point in history, the purpose of prescribing it once language has changed eludes me. What is the problem with language evolving? It does it all the time, and no amount of rules will stop it. The only thing these kinds of rules accomplish is making people feel superior and, oftentimes, look silly. It's almost a certainly that any of these rules that people keep bringing up is routinely broken by Shakespeare. He must have been a terrible writer. Here are some examples I dug up:
"No, I will be the pattern of all patience; I will say nothing."
"I will live in thy heart, die in thy lap, and be buried in thyeyes—and moreover, I will go with thee to thy uncle's."
"I will speak daggers to her, but use none"
"ourself will mingle with you and play the huble host"
"I will have such revenges on you both,
That all the world shall--I will do such things,--
What they are, yet I know not: but they shall be
The terrors of the earth. "