I would have thought adding two competing nighttime options at Disney World would extend a visitors stay on WDW property? Sure, some people might have to choose between spending the night at MK vs AK, but there are more visitors than ever now that are choosing to spend a day or two off-site. Having offerings like this would increase the chances that guests (non-local in particular) spend another night at WDW, rather than at the expense of one park over another.
During Christmas 2016, DHS sometimes only ran with one Fantasmic show which ended around the same time that the Holiday Fireworks at DHS started, forcing guests to choose between the two. This meant that guests who were only in town for a week or so, in theory at least, would go back to DHS for another evening to catch the other offering.