The loss and subsequent banning of maps and map users made nearly everyone I know stop playing, especially in the total absence of an official replacement or a compelling reason to continue. Hearing my map ding was enough cause to make me leap out of my chair and run to the nearby park at midnight to catch something I wanted, and there'd usually be the usual lure-users and a few map-users out there at the same time.…
This is on the money. Niantic made it harder and more frustrating to play their game, and people stopped playing it. They have utterly failed to understand what their playerbase wants, and people are falling off. The maps really drove it into overdrive, especially with the presence of "nests", because it allowed people to figure out where they needed to be, and would then congregate there. By taking out the maps, Nia…
You're phrasing this almost as if Niantic wanted there to be third-party mapping apps in the first place, and then changed its mind.
The game Niantic released doesn't have mapping, and didn't intend to have an open API. This can be read as an implicit statement that the game design that Niantic thought people would enjoy playing, doesn't involve knowing where pokemon are.
Presuming there was at least one actual game designer involved in creating the game, there's likely already some sort of effort-reward cycle built into the game that doesn't involve mapping. It might not be one you like playing, but that probably means that you were not the audience Niantic designed (and playability-tested) the game for.