As explained in this article in The Guardian,
Boris Johnson has written to the EU suggesting the backstop could be replaced by some form of commitment to prevent a hard Irish border in his first major move to explain the UK government’s new position to Brussels.
As far as I have understood from other sources, the backstop is not the first choice of a solution to the border of Ireland, but a fall-back option to be used after no other solution has been found.
In the letter sent by Boris Johnson, included in above article, the British Prime Minister writes:
This Government will not put in place infrastructure, checks, or controls at the border between Northern Ireland and Ireland. We would be happy to accept a legally binding commitment to this effect and hope that the EU would do likewise.
Since the backstop was not intended as the first solution anyway but merely as a fall back option, and since the PM in above letter promises to not to erect any border infrastructure, what is the improvement gained by replacing the backstop with a "legally binding commitment"?