Hobbit was in similar shoes as FB. It too came years after the original story ended, they were both prequels, Hobbit's story wasn't that much of a stretch compared to how poorly FB crew was mentioned in HP movies, an army of fans backing the franchise too. Hobbit had a decline, though much smaller than FB, between the sequels, and it earned significantly more WW than both FB titles. Yet still there were questions asked why was it filmed and its necessity.
It was crazy to expect LOTR numbers but still a decline in profits wasn't expected, same thins is happening now. WB can't make FB3 with the plan they have now, they will have to make some radical changes. Audience "endured" Hobbit for its three title run, there's already market saturation to some lesser extent present with FB and it still should have three more movies.
You're probably right with the multipliers, but you can't argue WB expected more from FB2 and it was a disappointment. If it was a standalone movie, or the final episode of a franchise then a movie earning +600M would be a success. But the engine is already slowing down and we're not even half way there yet. That's the problem here.