https://www.nationalreview.com/corner/founding-era-antislavery-and-the-overheated-freakout-over-tom-cottons-history-of-slavery/
The Founders did have a plan to abolish slavery; it just didn’t work out the way they expected.
As John McCormack notes, Tom Cotton may have been awkward in his phrasing, but there is nothing shocking in saying of slavery, “As the Founding Fathers said, it was the necessary evil upon which the union was built, but the union was built in a way, as Lincoln said, to put slavery on the course to its ultimate extinction.” Jonathan Chait writes:
Cotton seems not to be saying that slavery was necessary in order to get slave owners to accept the union, but that it was necessary to the “development of our country.” Here, oddly enough, he is recapitulating one of the most important errors in the 1619 Project itself.
There are two ways to read “necessary”: that slavery was necessary to build the country, or that tolerating the pre-existing institution was necessary because nationwide abolition was politically and perhaps economically and socially infeasible in 1776 or 1787. I agree with Chait that the 1619 Project is off-base in claiming the former; I do not read Cotton as saying that, and the people who are jumping on him over this are, it appears, just people who already hate Tom Cotton.
The formulation that slavery was tolerated as a necessary evil at the time of the Founding, and that the Founders expected (overoptimistically) that it was on an inevitable path to extinction, is a fairly standard one, and mostly an accurate way of putting the more complicated story of Founding-era slavery and anti-slavery into a nutshell. It most accurately captures the views of the Virginia Founders (such as Washington, Jefferson, Madison, and George Mason), who saw slavery as wrong — unlike John C. Calhoun and his followers in a later generation, who framed it as a positive good — but were unwilling or unable to face the effort to end it. It also accurately captures the view of anti-slavery delegates to the Constitutional Convention, who concluded that it was not worth breaking up the new nation in a vain effort to force the South to abandon slavery immediately.