There is only so much you can do to extend an existing field of knowledge.
Exponential growth in knowledge happens only when some new field of knowledge is discovered, and almost everything is unknown in there.
A good example was the discovery of electromagnetic waves. Before their discovery, people didn't even know that such waves existed. And of course, there was a lot to discover, once this field was opened up for research.
Running out of ideas is inevitable, when you are just trying to extend existing fields of knowledge. The only way to keep ideas going is to discover new fields of knowledge, where almost everything is unknown.