Visual theater isn't likely to disappear, but TVs as we know them..yup, they'll be gone within 20 years.
Yes. Network television is on the way out. Ultimately "on demand" entertainment will take over. However, I disagree with your assessment it will be within two decades.
The only roadblock at this point is the infrastructure necessary to make the technology completely ubiquitous, and by that, I mean completely word wide. Otherwise there will still be some form of network television someplace.
The cost and manpower necessary to accomplish that is extreme, and will certainly take a significant amount of time. Heck, I don't even know if it can be accomplished in the industrialized countries within twenty years.