I then interpret this as if people playing with wine would switch to Linux, we would have only the expense to realize a new version of WoW, *next game*.
Unfortunately there are ongoing costs associated with developing a game client; it's not a one-time expense. I've talked with Linux advocates who think that it would be rather cheap to create and maintain a native Linux client, but I don't really buy that..
So is he paradoxically suggesting that to show how many people play with wine, and to ask Blizzard to create a Linux version, all those people should leave WoW until a Linux native client is released?
I think he's asking "would the people using the linux client be totally new subscribers or would they be people who used wine to play before?" Basically, reading between the lines, he's wondering if they can get away with having Linux users use wine, and at the moment, that seems to be the case.
Back when Blizzard's Warden program misidentified a bunch of cedega users as hackers, there was a fair amount of frank talk from cedega maintainers and blue posters (reps from Blizzard who post on the official forums) about cedega/wine and wow. Blizzard posters mentioned they would be tracking the use of wine/cedega (since it's possible to do so, though some douchbaggery from Microsoft made the wine maintainers put in some features that make it harder for programs to detect if they're running in wine). Also, cedega devs mentioned that they are in contact with Blizzard developers to ensure that cedega will run wow. Unfortunately, cedega has fallen by the wayside as their codebase atrophied, customer support became inadequate, and wine overtook it in compatibility and bugfixes, so I'm not sure what the status of all that is now.
Couldn't Blizzard realise a poll on their website/whatever, only for subscribed, asking if they would play on Linux if a native client was there?
They've done that before -- I remember filling out such a poll a few years ago. I don't really remember if it was for World of Warcraft, but it was a Blizzard game, at least.
I think they fear 5~10% of players would answer yes, thus forcing them to release a Linux client.
I don't think they would. At least, I have no plans to stop playing.