A dedicated control surface, like
a mackie HUI, isn't going to get you the mic pre's, EQ, routing capabilities and effects that a traditional board will.
On the other hand:
I'm a Nuendo user. I have been since it's inception of version 1.0
I use a digital mixer. A Yamaha O1V to be precise.
The O1V works great as a control surface for Nuendo. It integrates seamlessly, and has no noticable latency. It's on board mic pre's are rather lack-luster in my opinion though, so I use outboard pres.
It's effects units: EQ, Reverb, Compression, et al are, well, OK I guess, but I prefer outboard gear for most of that. Nuendo's Compressors, both broad band and multi band, are very nice and easy to insert into a track or mix.
With the right combination of digital gear, and traditional outboard gear, you can turn out some damn good sounding tracks!
So, is the traditional mixing board comming to an end?
I don't believe so. It, like all great equipment, will always have its place and specific use in the studio.
For most of us, it comes down to a matter of which road you want to take to arrive at your destination.