They wouldn't - this is how the Montgomery was closed, burst in 1936 and then the act of abandonment in 1944 was justified on the basis there had "been no traffic for some years"
BUT there still had to be an act of abandonment. The LMS got away with it because in 1944 no-one cared, even if they knew about it. That scenario is unlikely now - you can't so easily get away with no-one knows and no-one cares.
Going back to the original point, where rose off my sick bed to challenge @IanD assertion that Peel weren't obliged to reopen it. They are, or they need to formally close it. They will get a lot of opposition if they try and may not be able to do so.
In another thread on here there was a video featuring the Warburton Toll Bridge, which is owned by Peel. They had been charging 12p for the toll, the original toll adapted for decimalisation but not adjusted for inflation. When major repairs were needed they sought and got legislative change to increase the toll to £1 and modernise the payment mechanism.
Peel won't just walk away - they won't just throw money at it either, they'll seek a new deal