Even if all The people who play Valve games switched that wouldn't even be 1% of all PC users.
PC users or PC gamers?
If you're talking about all PC users then possibly, sure. But largely thanks to Steam and its ilk, PC gaming has been argued to be growing again.
If you're talking about PC gamers then you must be pretty mistaken, particularly with Valve running Steam today, Valve's own games are a lot larger than that in the PC gaming market.
And the other point? Well quite possibly - I'd think EA and Activision are quite conservative - but then again they can just sit back and see how well Valve does on Linux, and if it does well they'll have a trusted distribution channel via which to dip toes themselves.
In any case having now noticed your username/nick I'm not going to continue replying to your comments, as it seems you've only registered the account to troll.