There may be no upper bound on the absolute amount of available resources, but there is still a limit on how fast you can expand.

Consider the following maximal example: If all of Earth's resources are exhausted (whatever that means, I don't rely on any exact definition), we need to head outwards into space. Because our expansion is limited by the speed of light, we can at most cover a distance

  r(t) = c * t

and thus a volume of space

  V(t) = (c * t)^3

where t is the time spent travelling through space. The amount of available resources (assuming an even distribution of resources on the scale of the universe) is proportional to V, and thus to t^3.

Cubic growth is much much much much less than exponential growth. In fact, the exponential function can be shown to grow faster than any power function.