Are publishers making less money on textbooks now?
To be honest, I'm not sure even to what extent a book can be seen as profitable separate from the online tools because of the ever tighter integration between the two. A number of textbooks in newer editions have started offloading (er, onloading? uploading?) activities that would traditionally be in the textbook to online systems.  This is especially egregious with audio or video activities that used to come on CD/tape/VHS/DVD, and now are only accessible online (those materials are important for us in foreign languages).

And to really drive home the point that they are considered to be a single unit, I've used at least two textbooks now in the past few years for which the physical book + e-book + online tools cost less than the physical book alone.  

So I guess in that sense, you could say that the online tools are where the money are (because that's where they're pushing people), but it's not like they could discard the textbook and just sell the online tools, at least in my field.  A good book can go without online tools, but the other way around isn't so true.  

Now there is a trend to goto single-use sells, and again, it's easy to conflate that with the online portion (and hard to fully separate out the economics of it without detailed numbers none of us have): they've always been able to grab people on single-use sells via workbooks, but now I notice there's a small trend to try to integrate textbooks with workbooks in a single print to reemphasize the formers' single use nature, with or without an online system.