Media queries are still necessary to lay out the page you've described. The primary containers of your pages all need to move around based on the size of the top-level container.
If you could only rely on your parent, that would also be problematic - the size of a component would be dependent on its chain of parents. If you were reusing the same component within further containers, say 3 different pages, you might want two to look the same despite a size difference, and element queries don't help you anymore. You just need a .testimonial.compact class, have slitghtly different media targeting, and now you have two types of testimonials that you can use on further pages.
Element queries could also easily create strange loops that cause the threshold to be crossed recursively, if the parent bases its width on the child. This is the case with inline-block elements and the property-which-must-not-be-named.
If you could only rely on your parent, that would also be problematic - the size of a component would be dependent on its chain of parents. If you were reusing the same component within further containers, say 3 different pages, you might want two to look the same despite a size difference, and element queries don't help you anymore. You just need a .testimonial.compact class, have slitghtly different media targeting, and now you have two types of testimonials that you can use on further pages.
Element queries could also easily create strange loops that cause the threshold to be crossed recursively, if the parent bases its width on the child. This is the case with inline-block elements and the property-which-must-not-be-named.