I'm sure when the 6D2 comes out later this year, the used market will be pretty good for 6D bodies. So you'll have to decide *when* you need a new camera. The 6D was around $1800-2000 when it was announced, and I'm assuming the 6D2 will go for about the same.
The 5D2 might be a good example to look at as a price floor. Right now the 5D2 is around $800 on the used market, and the 6D is around $950. So the price of the 6D will drop a little bit, but probably not by a lot.
Image quality of the 6D is between the 5D3 and the 5D4. Focusing speed is the biggest drop from your 1D3. I've heard focusing speed is similar to the 5D2 I have, which is faster than the 450D I upgraded from, but isn't anywhere close to my 7D2 or what your 1D3 can probably do. I used my 5D2 once for a college football game, and it was okay, but I had to work a lot harder to time the shot and focusing.
I don't have any direct experience with the 28-75, but from what I've read the megapixel increase from your 1D3 to a 6D might start to show the sharpness limitations of that lens. Your 70-200 will keep up with the increased resolution just fine.