I don’t mean to be a doomer - and I know that people said this when AQ40 came out - but I think that Naxx is the straw that will break the camel’s back.
For starters, Naxxramas is an exponentially longer raid. In the sense that it will take dad guilds multiple nights to (maybe) clear it, and it likely won’t get much better timewise for a long time. The meme that casuals need a nap between pulls is partially true; it’s difficult to get them to push when it’s getting later, or after many wipes.
Secondly, the raid is rather intensive consumables-wise. And the more casual you are, the more costly the raid is because the more you die and the longer it takes to clear the more you have to consume. Casuals will likely run out of gold faster than they can farm it, especially considering my previous point.
Then, there’s the learning curve. Naxxramas is commonly accepted as a turning point in WoW raiding. It’s that point in time where encounters become much more than just standing there casting your three-button rotation, and where powering through harder encounters (Sapphiron) with raw DPS isn’t necessarily a viable strategy anymore. We know from previous experience that casual guilds struggle with mechanics, coordination; the (casual) Classic guild I’m in right now still needs constant strategy reminders in AQ40, even though at this point we have dozens of clears. They still make stupid mistakes, some which will be exponentially more costly in Naxxramas.
Finally, there’s the fact that with TBC oh-so obviously over the horizon, there’s just no real point in doing Naxxramas, if not just to experience it.
99.9% of players won’t be aiming for world firsts (where Naxx gear is crucial, as for most classes it can carry you well into T5 content), and it’s unlikely you’ll get a full T3 set’s worth of clears for 40 people before the burnout starts to set in and it becomes more and more difficult to get warm bodies willing to spend the time, effort, and gold into raiding for gear. For many classes, most of the Naxx gear will almost immediately be replaced by gear from heroic dungeons and reputation rewards. While it’s true that some classes get gear that will carry over until T5 and even T6, you still have to convince other people to raid in order to get those items.
To be clear, I’m not saying to quit Classic WoW and that raiding in Naxx is a waste of time. I just don’t think that beyond experiencing the raid purely for fun, there’s just so many hurdles for casuals to overcome that it may not be worth the energy invested. I truly believe this is what will end up breaking up massive parts of the community, and turn Classic WoW into even more of a TBC waiting room than it already is.