Besides staff, what costs does an existing mission incur? That would have been a useful piece of information.
A mission's budget accounts for its share of use of the Deep Space Network, control facilities, computer access, engineering consults, etc. In the case of New Horizons a big part is telescope time (on the ground and in space) trying to find another target. Despite the feelings of the author, a major factor in the decision is that there's nothing for it to do. Enough observation has been done along its path that any body down to half the size of one of Arrakoth's lobes has been precluded to a very high confidence, and its window is only narrowing. It's not like a Mars Rover where there's always one more rock and one more sample as long as it can still move, it's hurting through the void with limited ability to change directions.
Worth saying there's a bit of "military math" going on where the money will get spent on the same things either way. Budgets show what missions benefit from spending but cutting a mission doesn't actually cut the spending. Nobody is losing their jobs and the DSN will cost just as much to run, but New Horizons will get less people and use less bandwidth.
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