IronWolf
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Upon the release of 4E, did Paizo gain some customers that WotC lost?? But, and this is important, anyone who thinks that WotC released 4E expecting 100% customer retention is being rather unrealistic. They knew that they would lose customers when 4E released. The amount of money that WotC losing to Paizo is probably rather close to insignificant when compared to WotC D&D revenue as a whole.
WotC certainly should have known they would lose some and I suspect they did. There are people that don't like change and wouldn't accept whatever was sold as 4e, even if it was just smaller tweaks, much less a major rework.
Where I think Paizo causes the most surprise is that now, all those people who did not move to 4e have had their choice reaffirmed. This is a factor I figure WotC was likely not expecting. It is quite possibly WotC knew some people wouldn't move on to the new system right away, but over time as the support of 3.x products dwindled people would begin to move just because 4e was the supported system. Instead, Paizo filled a void and those that remained with 3.x had someone that released compatible material and a ruleset that was much more inline with their idea of how D&D should be.
Of course what this means in terms of sales dollars versus sales dollars we can only speculate.