They have a billion Tridents in place, so would have to sell both combined units and solo units which would drive up costs. They already have the design from the original trident, so they save a bunch of money by repurposing it. Having two units also means you don't lose all the testing/control if one borks, as well as not forcing the user to test 5 things if they don't want to. I don't really care about the Ca and Mg results (thought they have been helpful when trying to determine if a salinity issue was really happening) and would have been totally happy with just an Alk version.
I am not sure that this tester is really needed by the hobby at large, except to automate and make consistent a test that is a real PITA - nitrate. That said, I am not sold on the utility of testing nitrate and phosphate regularly anyway, but lots of folks think it is critical, so they will sell a ton of these. I do think we will find some really interesting info from the NP, just like we did from the original trident.
I was about to call you a Neptune shill but I read further and totally agree with everything you said after the first couple sentences!