Decades?
I don't think so.
People forget that the revolution started by the iPhone is less than a decade old (2007).
I don't think anyone could have predicted this pace of advancement and shift in terms of how we consume media.
Those business models are not easy to change nor is the in-built infrastructure. Look at how long it took for the music industry to embrace MP3. Look at how long it took them to ditch DRM. Look at how long it took them to get to streaming services when we were all streaming MP3's on Napster way back at the turn of the millennia.
They are moving about as fast as you can expect these giant tankers to move.
The iPhone's affect on cable wasn't just in how people consume media, but also where they prioritize spending their cash. It's hard for many people to justify the high cost of cable and the high cost of a phone plan. Something had to give, and for many that appears to be cable. Probably because you can recover what you lost on phone more easily than you can recover what you lose by giving up a phone with TV.