They were still wrong, and frankly, they knew it.
If by "they" you mean Southerners in 1787, then yes, of course that's right.
At the time, virtually everyone considered slavery a "necessary evil" which should be abolished "some day".
But "some day" never came in the South, and by 1860 the old "necessary evil" had largely turned into a "necessary and unquestionable good".
Still, I can't think of a time or place, in 1787 or later, when Founders seriously considered that slavery was a national responsibility, which might be abolished nationally.