I don't think anyone is suggesting that teachers can't dynamically adjust their attention across students as the class progresses based on grades. But at the same time teachers should not just be the pawns of parents. A parent shouldn't be able to just demand extra credit work for their child. This sort of special exception is something that should only be made by the teacher, at the teacher's complete discretion, if they legitimately think there's a truly good reason for treating that student differently.
Giving students the chance to learn is important. Expecting them to actually master some skills within a given timeframe is also important. I have a relative in a European system in which changes have been made to allow students to take tests on a class for the next two years. The students are not better off -- it sounds miserable for all.
You also say that a student who does poorly didn't put in the time. In some cases yes, but broadly it's simply not true. I've analyzed LMS data at large scales: Students who get F's don't put in much time on average. But students who get D's, on average, are putting in just as much time as the students getting A's and B's. The F students I can't do much about, with some notable exceptions, they mostly don't want to help themselves. But the rest are teachable.
For me and my like minded colleagues, it's not that we expect ourselves or others to do more work. It's that our vision of what the job's baseline work requirements entail goes beyond a rigid grading process that implicitly assumes that learning takes place in discrete bits of time, uniformly for all students.