Do we know what kind of data is steam spy using? My guess is that concurrent players count is a very important data point for Steamspy and that a high engagement game like MHW might skew the projections. I could be wrong, though.
Is not like I have hard data, but we do have the 2-3M launch period sell through. To reach the 5-10M, MHW should have keep at an extremely strong pace. But, for example, if we look at steam sellers right now, is at a respectable 16 place. Which means the game is still selling strongly but not to the point of doing multiple millions per month. I could be wrong, though.
Actually this is the first time i see the game, since release, not being in the top 10 global top sellers (and almost all the time it was top 5.