Bottom line should be:
Replace D coaching staff and see what happens. If D still stinks and LU suffers, AC has at least bought himself a couple of years to prove his new hires can work.
Don’t replace D coaching staff and bank on them to do well. If D improve and LU beats LC and is in the PL hunt at least as well as this year, AC stays. If not, he probably should go for sticking too long with the D coaching staff.