Well, there you go. TNA and other feds aren't as big as WCW. NJPW is on the rise, and hopefully they try to cater to their non-English fanbase. Thankfully I have a handle on Japanese and can read the names and parse the language, so I use NJPW as enhancing my language learning abilities. Most wrestling fans won't get that from NJPW. That being said, I think NJPW is a big niche. Any foreign product has the potential to be niche - and usually is - when it comes to media and entertainment aside from football (soccer). I think it'd be very hard to convince MOST people -WWE fans or not - to watch NJPW. They just are't that hardcore into the hobby. But if their favorites went to New Japan, I don't think it'd be that far away.
I still think that wrestling is a business driven not by company name, but star power. WWE just has all the relevant stars - in the English speaking world - right now. Let's say Punk went to NJPW. I bet you New Japan would have an increase in watchers.
I just don't think wrestling fans have much brand loyalty. In the Attitude Era, everyone I knew who watched wrestling watched BOTH feds and we flipped the channels during commercial breaks. The ratings for both indicate this was true for many people and not just my limited circle.
While you're right that WCW built a fanbase over time, think of the main factors as to why WCW blew up: star power. They got Hulk, they got Nash, Luger, Flair, they got Hall, they Rude, and Hart, and all these stars from WWF jumping ship. Of course they blew up - they stole WWF's fanbase. WWF started to get an edge when many WCW stars jumped ship to WWF. I stopped watching WCW by 2000 simply because all my faves had went to the F.
When Impact went to Monday Night, they indisputably had more star power than WWE.

it might help to empathize with what the fans want.