Brits call it The Rebellion.
They are correct, of course. The colonies were largely self governing. When we rebelled the same guys were doing pretty much the same things, just without a guvnah..
Bit of a stretch to call that a coup.
We weren't talking about 1776; but the 1787 Philadelphia Convention. The OP seems to think that the Constitution of 1787 was an illegal seizure of power.
The problem with his argument, however, is that he used the term
coup d'etat, which, by definition, is the infiltration of some key segment of the permanent state apparatus--the military, the intelligence services, the police--to displace the in situ government from its control of the state. The U.S. was a loose confederation of states, with no integrated military, intelligence services, or national police force, and certainly nothing whatsoever of a state bureaucracy. Also, there needs to be some sort of center of power in which it takes place to displace that power. The "United States" per se was little more than a shifting, unelected, unicameral Congress which was used to coordinate the activities of the thirteen independent states. It moved around constantly. The centers of power in 1787 were the state capitals.
In short, you cannot have an actual coup d'etat take place in the U.S.A. in 1787 in ONE location: it would need to be carried out thirteen times, in thirteen different locations, and involve force--or at least the threat of intervention by force--to be a coup. Of course, you could argue that the legality of the manner in which the constitution was approved was in violation of the Articles of Confederation. But it wasn't an illegal seizure of power, or really a seizure at all. All thirteen states approved it (eventually), and nobody ended up leaving the Union, or trying to leave the Union, for at least 71 years (1860) after its establishment. The states that were reluctant to ratify it still ended up doing so, since they were promised a bill of rights would be proposed by Congress within a few years--which of course it was, and all thirteen states ratified 10 of its 12 proposed articles within a year or two after its passage by Congress.
The worst you could call it is a Thermidorian Reaction. But such reactions are often necessary in revolutions to keep them from ending up in disaster.