Call me an optimist, but I am not worried, here's why...
The two potential competitors are in-house modeling becoming easier, and overseas firms. I realize some clients will inevitably go one, or both of these routes. However, the majority of your local clients probably will not.
Keep in mind who your clients are. The majority for me are small to mid sized local firms. If you primarily work for clients over the internet and can't offer any personal service, then you are more likely to lose some of them. But this type of client is less stable to begin with. However, if your clients are local and benefit from having you nearby for flexibility, peace of mind, etc... then they will be less likely to do either.
I have lately been trying to broaden my market by offering more types of services (ie. conceptual for schematic design, simple interactive presentations) as well as targeting all scales of firms. I have found smaller firms to be some of my best clients, and small jobs are often my most rewarding.
When more 3D modeling is done in house, several good things happen for us. Clients are already thinking of 3D and presentation, instead of waiting to the last minute and calling the old hand renderer (which many still use) Also, when I am provided with a 3D model, my clients typically are able to afford higher quality work, or I can charge less for a great rendering... and they can afford a great rendering more often.
While I think all architects should be using 3D for design presentation (and therefore including this work in his fees) usually these images do not cut it for press release. Even if the architect can produce better results given enough time, he is not interested in working for free. And rather than 'nickel and dime' the client for a couple thousand more in fees, it is often more convenient and respectable to offer to get a professional artist. Clients are already accustomed to this relationship, and it can help keep the architect from being 'used' to create marketing material.
Chris