Obviously Anakin was forced to be fitted with his suit to save his life after his catastrophic injuries on Mustafar, but what if Sidious always planned for this to be his fate regardless of how his duel with Obi-Wan had ended?
Let’s assume for a moment that Anakin won on Mustafar…what would have been his next step? It’s heavily inferred that he planned on returning to Coruscant & brashly confronting Sidious for control of the new Empire & it’s also implied Sidious fully expected this.
So next question…did Sidious originally intend for Vader to be his eventual replacement? What do I think? No.
First & foremost, Sidious was a pathological liar & manipulator who would say & promise anything to get the result he wanted. Secondly, he truly believed he was the prophecied Sith’ari & the true culmination of the Sith Grand Plan, you really think someone that narcissistic ever intended to obtain a serious apprentice who could actually surpass him?
Look at the apprentices he trained…while exceptional in their own ways, none of them could ever have overtaken him…except for Vader, who conveniently suffered crippling injuries before he had the chance to confront him in person.
Here’s what I think Sidious’ plan for Anakin actually was: he’d feign being surprised at Anakin’s attempt to kill him, lull him into a false sense of security & then cut him down in an overwhelming display of raw power. But he wouldn’t kill him…just hurt him so badly he’d need something like the suit in order to stay alive due to the injuries inflicted on him.
In the end, Sidious gets what he wants regardless of how Mustafar played out…a new ‘Grievous’ to lead his armies & act as his enforcer while being too weak to ever be a serious threat to his plans.
He never intended on being replaced, he just wanted the most powerful being next himself comfortably under his thumb.