One of the things that seems to be misunderstood about The Force Awakens, including the characters themselves to be fair, was the idea of the map leading to Luke Skywalker. Technically that was correct, but that implies it was created after the Jedi's self-imposed exile. Instead, the map - and the location - already existed long before Kylo Ren had massacred Luke's Jedi-in-training. Indeed, the majority of the map we see in the movie is contained within R2-D2, which he downloaded when he plugged himself into the Death Star way back in A New Hope. So that means that the Empire had a map to what is believed to be the first Jedi Temple, for reasons unknown. That's something that could well be picked back up in Episode VIII, especially going with theories of Snoke wanting to destroy the Force, and suggests that Ahch-To is indeed a very ancient and powerful place.