Regardless of set of numbers, you're still talking about at least 20-30% of the 360 market not being connected if dirtyvu's numbers are right.
Hard to see MS telling 20%+ of their current market to go pound sand and turn away all that potential money in hardware and software sales.
It wouldn't make much sense, but Microsoft has done a lot of questionable things. I think there's a chance they won't, but a nagging thought in my head is saying they think they'll recoup the cost with those purchasing it solely for a media box and not a game console, and on those who may purchase it with a contract (if they have that model) even if they don't have an online connection.
Then if they ditch points and go with a $ system, those point cards sales will go on sale as much as PSN cards do, so basically non-existent making them even more money. Even more so if they increase the price of Live which is entirely possible.
So while giving the finger to a chunk of their customers seems like a poor decision, they may think it's worth it if they can trick or manipulate the sales elsewhere.