Just to set the record straight:
P-8 is a multi-mission platform, depends on what kit you choose to buy to stuff into it. This is consistent with how P-3 and EP-3 all added kit for multiple missions as time went on (and a lot of kit got smaller)
Beyond ASW, missions for MPA include SSSC, ESM ELINT, and other mission areas. Has for about a quarter of a century in the USN. P-8 had to be able to carry that in order to get the nod. I think the "modular" requirement is where you can best tailor the kit in an MPA to meet your given needs. (No, I do not work for Boeing, nor the P-8 program office).
The argument that P-8 is a single mission silver bullet is incorrect.