And, are ideas getting harder to find?
Perhaps a useful distinction is between a) the more theoretical question of whether the nature of knowledge itself poses bounds, and b) the more practical question of how our culture/population affects growth rates (via quality adjusted research hours OR bureacracy).
As pointed out in the last paragraph, there are no guarantees of growth - exponential or hyperbolic. When it occurs, exponential growth is the exception rather than the rule.
There are many historical and present cases under b) where growth is bounded or negative.
As to whether knowledge itself is bounded... to argue that one way or the other, the articulation of a theory of knowledge is required.