The problems the author has still can be solved with a flexbox, it's just that a grid offers an alternative. There are justify-content and align-items properties that with a properly designed nested arrangement of flexboxes can help to create any layout that a grid can create.

Template areas make responsive designs trivial. You just redeclare them for desired viewport sizes and that's it. Also grid gives you more independence with regard to content position in the source.

Yeah, but you need to create container elements to make it happen, cluttering your HTML and making it more difficult to swap elements around the grid as needed.

That's true, although I wouldn't call containers "clutter", they have multiple uses. But yes, flexbox though easy and logical, still requires messing around if you need to reposition or change the configuration of things. It's not awful but probably slower than css grid. For me, I don't usually need to switch things around too much with layout, I sort that out before build.