I will link some sources, but please cut down on the rhetoric (I don't owe you anything, your illusions of reality are your own business, so next time you demand something of me instead of ask, I will ignore the request).
Here is an approximate number of raiders for tier 16 (SoO, bigger than any other raid tier after it) from wowprogress:
http://www.wowprogress.com/stats
The absolute highest number is the number of players with just one kill in normal - 1 mil. That's for all of US+EU+Asia, although we have to ask wowprogress as to how complete the numbers for Asia were at the time. The number correlates well with other numbers, such as those published by MMO-C. Again, that's all people with just one kill in normal difficulty - not even one clear, one kill.
Now, the size of the 3v3 ladder in the same season:
http://www.arenamate.net/arena-rbg-t...fs/archive/s15
That's 330k for US + 375k for EU = 705k for US+EU. Add Asia and we'll have about the same 1 mil as above.
You can see that the numbers are completely comparable.
The number of mythic raiders is significantly smaller than the number of people who do rated arenas. It is more like the number of people who are 2000+.
Yes, in Legion, PVP is in such a bad shape, that PVP numbers are dwindling. With time, the number of players who do rated arenas might reduce to 1/2 or 1/3 or even 1/10 that who raid. The number of people who do any PVP at all might similarly reduce compared to the number who do any PVE at all (talking about both activities at max-level). That's exactly one of the big issues with Legion.