With regard to phone replacement, yes some do replace frequently..on the other hand the huge surge in smart phones can't be sustained indefinitely. They will hit a wall where they can't do a lot with the technology, screen sizes can only get so big, resolution again big jumps but we reach the point of "good enough"
Some users will replace a broken phone, some upgrade but it's not sustainable long term.
A similar thing will happen to tablets, screen res has improved, lighter and thinner designs but there is only so much that can be done.
It's actually quite close to the camera market, big surge when the technology was coming onto the market (circa late 90's 2000 odd), technology improved so did resolution/DR, bigger screens, more features.
Now we have a situation where there are not a lot of improvements happening, fairly incremental in many ways. I do expect new sensor technology..but still we have that wonderful thing called "good enough", people I know are upgrading a lot less bodies wise then they used to 6-7 years ago, once you've bought the lenses you need purchases tend to slow down, you might sell the odd one or try a new one.
Market saturation is an issue that can hit any product. PC industry (classic example), TV's (a sore point for some makers who are struggling)
Sometimes a product turns into a multi use one, such as the ipod, they still sell them..but nowhere near the huge numbers they used to. The phones do all that now for many people.
I'm not sure where the camera industry is going to go, it's not going away likely never will. But some makers are going to have to quit the market (there are too many out there), less models, longer updates. I think they will have a big full frame push as one last gasp and it might do fairly well. Already there has been a huge reduction in compact camera models, and makers moving to more enthusiast/premium ones for those who are actually interested in photography.
Will be interesting to see how things pan out over the years. It might actually be good for the consumer as makers are forced to try harder, make more meaningful updates, and probably better prices.