I made the point that no writer can assume this, ever. We have no way to control how any reader might interpret what we say, but we can control how we say it. Grammar is control, and to ignore it is to toss away one of the best tools a writer has.
For a weak analogy, I mentioned someone who might work for days and days on a beautiful hardwood floor, only to leave a deep hole in the middle of the room. "People can walk around it," he would say, but is that a safe assumption? And what happens in the dark?
I always assume that anyone who takes the time to read my stories will know more about English than I do, and will have more important books lurking in the background waiting to be read. If I allowed myself to slip, if I allowed myself to write without my full attention, any clumsy phrase or ugly clause would be as obvious as a hornet's nest on a sidewalk, and I would lose the reader's trust.
In that sense, grammar becomes more than just a tool for writing; it becomes a promise to the reader. It says, "I respect your knowledge and your taste. I appreciate your time. Let me use that time with competence and care."