Well, I think the source for a lot of users like myself isn't the decision to cut updaters off. I acknowledge I always saw the business aspect of the equation, which is why I frequently just browse the site with ads blinking in my face.
But, there are a lot of people out there like myself that help guildies with things like this and now we're being flooded with people asking for help in manually updating mods they need to raid that they were barely able to install the first time. And the only choice we have to save our sanity is pointing them to Curse's client.
Every person that switches to Curse's client is going to be highly unlikely to return even after your updater is finished and that's bad for revenue (but good for bandwidth?!?). People are sheep and they are being herded to Curse and they will stay there until the grass is gone just like they did with WoWAce and now WoWMatrix. In the meantime, you might have pretty green pastures, but nobody's coming to graze. And it won't matter that Curse's grass is half astroturf.
I hate pointing out issues without suggesting solutions, so...
Consider allowing updaters to work again until yours is done
Since bandwidth was the issue you posted and not revenue loss (yes, I know that's a part of having bandwidth), let the updaters work for the big tickets like ORA, BigWigs, etc.. Just give those updaters the bare necessities if you can afford it to keep the users using the updaters they have instead of switching just to Curse.
By doing this, you could reverse the mass movement to Curse's client and maybe even grow your userbase, because people already have updaters working will stay with it until it doesn't. Remember those sheep from above?
This is a small set of mods, but a large number of users, I get that. It takes bandwidth to push a smaller number of mods to a larger number of users, but I'm thinking longer term in that by not keeping those users on their existing updaters and off Curse, you'll end up with more bandwidth then you'll ever need because the user base is all at Curse.
This would also take some stress off your developer so he doesn't end up pushing himself to get a product out that isn't ready. Because that will just further exacerbate things if it comes out and is functionally behind Curse's client.
Right now, Curse has all the advantage, but I still think WoWI has the superior product. I don't want to see WoWI lose out. I know this is probably keeping you up at night and causing you gray hair. So let me end by saying that I wouldn't be taking the time to type these things out if I didn't appreciate the site and the effort that has been put into it. That I admire the abilities of the authors that post here and really like to anonymously and quietly troll about. I don't want to see it fade out by losing out to MegaMart, er, Curse.
Further, all those attacks on you and the site by angry people, just try to understand that even those folks wouldn't be so angry if they didn't value the site so much. The magnitude of their anger reflects just how much they feel they've lost and that's a testament to the worth of the site and its community.