To be fair, that nominative $15 (in the US) has stayed where it is because it's subsidized by the WoW Token in most markets - and the token was making so much money in China that they abandoned the monthly subscription fee there entirely. Even just in the US, highly competitive players like Mythic raiding guilds and upper-ranked Arena players tend to max out the amount of gold they're able to rake in via the token each week, and even go into gold debt with other guilds, all so they can buy BoEs, extra consumables etc off the auction house to stay competitive. And even a number of more casual players funnel cash into the token to obtain large gold sinks like the AH mount.
Even putting the token itself aside, there is also the in-game cash shop with exclusive cosmetics, and paid services like server/faction transfers, renames etc. In short, I would wager the true average monthly price for WoW for most people even just in the US is well above $15/mo, and goes up even further as you get into the more competitive echelons of players for both PvE and PvP.
Way to make your stance sympathetic
Yeah for me, the value of any price increase will be dependent on what I get (as I laid out in greater detail in my reply to Segev.)