Okay, we've probably all ran into this either online or in person. You know the type (heck, maybe you ARE the type, LOL): Know-it-All Magic:The Gathering gamer who can't stomach the mere possibility that another new card-game is in town (Warhammer:Invasion) and is stealing (gasp!) a good chunk of attention away from his or her precious Magic.
What's sad is that there's plenty of room for a huge variety of card games out here. There's no need for any Magic fan to consider Warhammer: Invasion as a threat - they are both different games that fill a different niche. Though these former (or current) Magic players will try to convince you that Warhammer:Invasion is nothing more than just Magic with different clothes on. (Something we can all agree isn't the case. Similarities abound but there are far more differences than similarities.)
My suggestions for dealing with this particularly annoying type of gamer :
1. Point out the specific differences between the games rather than the shallow similarities
2. Showcase the factors that make an LCG far less expensive than a CCG
3. Explain that their beloved Magic isn't going anywhere any more than the newer Warhammer:Invasion isn't going away any time soon, either
4. You could always just not bother even trying to go through the hassle of dealing with this particularly annoying brand of gaming elitism
5. If a disparaging know-it-all Magic fanboy comes around to denigrate W:I, just toss out some Black Lotus color copies and watch them scurry! It's fun!
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Unfortunately the reality is that there will always be a subset of gamers for any hugely popular game that feel that only their game and their game alone can occupy its niche in its own genre. You're probably not going to convince these gamers otherwise - they're a lot like the police horse in a downtown area with blinders on - they're probably never going to be able to see past their personal "blinders" to realize how awesome, cool, and unique this NEW card game is.