We overestimate the permanence of big things and underestimate that of the small. When a company is big and seems well established, we think it is less replaceable than the small mom-and-pop shop somewhere in your neighbourhood.
But getting big means more leverage, more overheads and it is also costlier to maintain. It is hard to imagine an alternative big thing to replace Google, or Nokia, or Kodak at those points when they are at their peak, until an alternative appears. And at no point before the big unravelling would the alternative seem like it is displacing what was thought of as permanent.
Fossil fuel is big, and we think baseload is hard to displace but the alternatives will slowly gain traction invisibly before you expect the collapse of the incumbent.