More like they pragmatically did what they thought was necessary to present a stronger front against the possibility of England changing it's mind about their secession.
But you dress up their motives very prettily.
They had already fought one terribly destructive war against Britain and were not willing to fight another, just as destructive, with each other over slavery.
Especially since most of the states in 1787 were still slave states. The "free" states would have been greatly outnumbered.
But in 1787 there was no possible way for Brits to "change their minds" about US independence, since they had already fought & lost a six year, very expensive war to prevent independence.
And with the French Revolution threatening, Brits were in no condition economically or militarily to take on another war against Americans in 1787.
Rather the need for "a more perfect Union" was clearly demonstrated by factors much closer to home, including:
For starters...