If you take the focus off of Cassie and Kang, and instead wrote it to center around the two title characters and MODOK (with a good character design), I could see the potential in this movie idea. Kang comes in after MODOK's failures, to trap Scott and Hope in the QR, which he admits to torturing/killing them when he sees the surviving trio in Avengers 5. This gives Cassie a heartbreaking backstory which audiences would connect to, and could make her a focal piece of Avengers 6.
Instead, they tried writing Cassie as the sassy know-it-all wannabe-Iron-Woman. They obviously can't write that character, so they should stop trying.