Of course people who WANT the stuff will buy the stuff, even if it costs $100. The issue is to convince people who are lukewarm, the majority of potential buyers out there. The hardcore fans will always fall, they are not the one FFG needs to convince.
As far as having friends who bought WFRP3 (or other editions for that matter) and can showcase it… ha ha! Right… like WFRP is such a well known universe and game. In the U.S., finding a WFRP group is like the Grail's Quest.
A Beginner's Box for v3 wouldn't sell people on the game. It might convince them that the basic system is good, but that's not the issue for most people - there are tons of good systems out there. The issue is whether the components are worth the hassle overall, and that's something you can only figure out by getting the core box and giving it the old college try. EotE doesn't have the "X-factor" of massive fiddly bits, so its Beginners Box is a more accurate representation of what you'd actually be getting yourself into after buy-in.
Also, as you say yourself, a Beginner's Box for WFRP wouldn't be able to cover the WH setting. So a WFRP Beginner's Box would only be showcasing the system and players would still have to do their own research on the setting. Everybody knows the LotR and SW settings already.