My prediction is that every behemoth of today will be tomorrow's Sears Roebuck, GE, West India Trading Company, etc. At some point, they'll become mired in bureaucracy. Enough incompetence will eventually rise to the top to allow competitors to pounce.
I'd bet on that, if I had to.
That said, I may easily be wrong, and I honestly share your concern about Amazon, Google, Facebook, Apple, etc.
In particular, I want a successful OSS phone competitor to Apple and Google. I don't think something as important as our telecommunication devices should be run by a duopoly. There's no freedom in the phone market the way there is in the PC market, and I'd really like to see that change.