You don't see that it can be done because your only experience is with a system that did it poorly. It has been done and it has been done well, just not here. (And yes, destreamed grade nine was done poorly. There weren't enough supports put in place so teachers were still gearing their lessons to the middle of the class. If destreaming is to work, it has to incorporate a lot of small-group instruction and guided independent study.)

Assessment shouldn't be whole-class most of the time anyway, so making it easier to administer a test to the whole class is not of primary importance. The idea is part of a style of education that doesn't work for many students, even the ones it appears to work best for.

You've got a level of privilege here that is skewing your ability to see the possibilities and the studies for what they actually say. The studies do not support streaming; they say that high expectations paired with high support can indeed lead to a high level of success for practically all students. Why would we settle for anything less?