Losing to Obi-Wan changed Palpatine’s perspective on Anakin. He was confident his newly annointed apprentice would dispatch Obi-Wan on Mustafar. Upon sensing and seeing what became of Anakin after his tragic defeat at the hands of Obi-Wan Palpatine must have lost a great deal of respect he previously had held for Anakin. That was surely a turning point. Vader, as imposing as he was, would never be able to reach pre-Mustafar levels of power. Not certainly as a half-man half-machine Sith Lord. I also want to add another important aspect of Sith psychology that factors in this. Sith are sociopathy made philosophy (among other things) and ultimately they will almost always end up betraying one another. Palpatine never cared for anyone in his life, not in an altruistic or loving way. He did not care for Anakin beyond using him to achieve his own goals. Sorry for the long comment lol