As someone currently doing something like that, I'd say it massively complicates the process of getting a coherent book finished this decade. The first issue is that people never deliver their chapters on time! Everyone's busy, and it's like herding cats to get a bunch of well-known people to deliver a manuscript, which is usually not the most pressing thing on their TODO list. (I'm guilty of it also.) The second issue is that if you want it to be a coherent textbook, vs. more of a collection-of-tutorials style anthology, everyone has to be on the same page about the overall approach the textbook is taking, and the chapters have to fit together closely. This is particularly important when later material builds on earlier material, and needs to do so in a way that works pedagogically (as is almost always the case in a CS textbook). Which requires a lot more Skype meetings and usually iteration on the chapters.
In our case we tackled it by having three primary "in-house" authors, at the same institution, who decided on the overall approach of the textbook, and outlined the whole thing. At least one of us was then also a co-author on every chapter; external contributors were invited within this context, to write part of a chapter in collaboration with us. This helped integrate the external contributions a bit more easily, and made sure they fit in to the book. And we had only about 8 external contributors. I would definitely not want to go all the way to having completely separate chapters written by different people and try to integrate them, and get the book finished before 2020.
The short version is: it's a good idea, but integrating multiple people's contributions into a coherent textbook is hard. If you take 10 people, and take 1 chapter each from the textbook they would've each written if they were writing the whole thing, the most likely result is not a coherent textbook, but 10 chapters that don't really fit together. Not just in the sense that you'd word things a bit differently, but that you'd organize the book completely differently and emphasize completely different material, so the result doesn't introduce material in a progression that's useful to a reader trying to learn the subject.