True, but the Constitution is not silent on the fact that some powers are reserved to Congress and other powers are forbidden to the states. States cannot be admitted to the Union without the approval of Congress, cannot change their status without the approval of Congress, by implication should not be able to leave without the approval of Congress. After all, the secession of a state does not affect only that state, it can have a negative impact on the interests of the states remaining in the Union. Shouldn't they have a say in the matter?
What you say is definitely a valid opinion, but you seem to be elevating your reasoning to the status of Constitutional law.