Treason against the United States, shall consist only in levying War against them, or in adhering to their Enemies, giving them Aid and Comfort. …The others are not black and white definitions, like this one is. Just because no deviation has had precedent does not mean it is illegal or unconstitutional; that is why we have a Tenth Amendment. (And no, the eighteen states’ recall processes apply to US Senators, not just state Senators.)
Conjecture and opinion is believing that there are other remedies outside of those provided in the Constitution. The Constitution gives the method of removing someone from office. You claim that since it doesn't specifically exclude other methods then those are left to the states. That is just not so. It's like saying the Constitution specifies the manner in which a state may be admitted, but since it doesn't specifically say that a territory cannot become a state merely by ratifying the Constitution and sending representatives to Congress then such a method is acceptable. It isn't.
The 10th Amendment says that powers not delegated to the United States are reserved to the states unless prohibited. The Constitution says that the ability to remove a sitting official is a power reserved to the United States, as outlined in the impeachment process. Therefore it is not a power allowed to the states.