>we become experts at many things, but at the cost of flexibility and speed of learning
It's partly because you are an expert at a unique combination of many things that you have great irreplaceable value. This is threatened by aging diseases even before you die.
Also I don't think that more knowledge makes it harder to learn.
This would be true if knowledge was purely about breadth. However it's really about explanatory depth, and depth ultimately wins. For instance, your mind has formed many connections between those aforementioned different fields of expertise, and some of these ideas reach into yet more fields.
It's partly because you are an expert at a unique combination of many things that you have great irreplaceable value. This is threatened by aging diseases even before you die.
Also I don't think that more knowledge makes it harder to learn. This would be true if knowledge was purely about breadth. However it's really about explanatory depth, and depth ultimately wins. For instance, your mind has formed many connections between those aforementioned different fields of expertise, and some of these ideas reach into yet more fields.