You mean ~500k in the last 5 months. MHW was already at 7.5m as of March 5, roughly 5 weeks after launch. Although we are due for an update since we only have shipment through the end of June, so ~500k in the 4 months after the first full month of ~7.5m.
Technically due to the combination of historically staggered western launches and traditionally higher sustained Japanese sales, it's done less globally so far after launch window than every game since the first PSP title. It's not at all incorrect to say the game appears frontloaded even by series standards.
Monster Hunter World(shipped+digital sales)
Release late January: 5 millions
Early February: 6 millions (+1 millions in the two weeks after launch)
5 March: 7.5 millions (+1.5 in the following 3 weeks)
9 May: 8 millions (500k from week 5 to week 13 last update we got)
https://www.vg247.com/2018/05/09/monster-hunter-world-8-million-shipped/
Even looking just at the time frame you comfortably picked as "after release window" to back up your scenario where it's preforming badly compared to previous games:
From roughly week 6 to week ~14 it moved 500k
Compared to the legs of the best selling MH games(looking at media create archives), from week 6 to week 14 they moved:
MHP3rd = ~368k
MH4 = ~282k
MH4U = ~268k while going through holiday season
MHX = ~177k
Add two or so more weeks of sales if you want or if I made some errors, and maybe add western sales data to MHX/MH4U if you can find any that far from release, but it wont change much. The 500k you're trying to push as "done less so far after launch window than every game since the first PSP title", is actually the best MH series have ever done that far from release!
But yea, it's preforming badly after release compared to the old games. ¯\_(ツ)_/¯