Your point is not wrong. But - NIU only cares about the football, and they're going to a better league (even after the changes). And their average road trip in-conference in all the other sports drops from 419 miles to 330 miles. The longest in-league road trip drops from 978 to Amherst to 516 for Robert Morris.
It's not huge - this is admittedly no $10,000,000 savings, but with all the other sports, it's going to be a greater savings than the cost for extra travel in the MWC. It will be interesting to see how attendance changes for all their sports...it's possible they also see a little bump in basketball (in addition to the likely notable football attendance bump) because the team will have a better record (as noted, the Horizon League's bottom is definitely scraping the bottom of the barrel).
It's not a life-changing travel savings, but it more makes up for the MWC football travel when aggregated across the other 12 sports.