General Discussion

Easy: because such a mechanism is provided for under the 25th amendment to address circumstances where "the President is unable to discharge the powers and duties of his office."

Where is the evidence that President is "unable to discharge the powers and duties of his office"? There is none. There is, however, ample evidence to the contrary. Even after dropping out of the presidential race, he has continued to discharge his powers and duties. He signs and vetoes bills, issues executive orders, meets with foreign leaders, both here and abroad, and so on. Poorly performing in a debate is not even in the same universe as being under anesthesia and literally not able to perform the actions assigned to the president by the constitution.

Your argument fails, miserably.