General Discussion

The Constitution says the President's term is four years. You're right that it doesn't say "and we mean four years, and you can't cut it short just because there was foreign interference or because the President concealed the extent of his polio-induced paralysis or because your astrologer recommends it or ...." But the absence of that language doesn't mean you can improvise.

There's a legal principle that long predates Donald Trump's birth: Expressio unius est exclusio alterius, meaning that the express mention of one thing excludes all others. Given that the Constitution provides that a President's four-year term can be cut short against his or her will through impeachment or through the Twenty-Fifth Amendment, established principles of construction mean that it can't be cut short in any other way.

With a normal statute, it often happens that some totally unforeseen circumstance arises. The statute produces a bad result. The remedy is to amend the statute, which requires only a majority in each house plus the President's signature.

When a provision of the Constitution is found to have that kind of problem, matters are more difficult, because of the difficulty of enacting an amendment. One example I can think of is the Eleventh Amendment. The Constitution didn't expressly bar citizens of one state from suing the government of another state in federal court. The Framers hadn't thought about this because everyone assumed that such a suit would not be permitted. In 1793, however, soon after the adoption of the Constitution, the Supreme Court held that such suits were proper. The amendment process geared up with notable rapidity, especially given that state legislatures back then held only short and infrequent sessions. The amendment passed the Senate by 23-2 and the House by 81-9, and the ratification process was complete less than a year later.

The point is that there was no way to deal with this unexpected circumstance other than through a Constitutional amendment. A government loaded with people who had written the Constitution realized that. It's simply not "obtuse" for people to tell you that the answer is the same here.