You see this in all other species, but we somehow think it's different for us. That remains to be seen.
If we lived in a world where every culture had achieved a certain level of education and development (and most all of them do want to get there), it's likely they would all have flat or declining populations. They might have to heavily subsidize reproduction just to sustain themselves and all this talk about building space habitats would be even more abstract than it is today.
The thing is if there's any culture for which that's not true, like Mormons, Orthodox Jews, some groups of Muslims, whatever. In a few generations they replace the cultures that don't have a lot of children like our own. Then expansion continues.
I could see expansion slowing even reversing temporarily, but it seems hard to imagine it permanently ending.
Anyway I agree that we can't say for certain whether population will grow or shrink. In fact in the context of this discussion I think that's the most important point. Many discussions about alien civilizations tend to assume that they'll evolve into huge multiplanetary populations with large energy/EM/whatever footprints. When in reality our most developed cultures are slowly declining in population. If an underground cache of brains in pleasure boxes is the end stage for most civs we will probably never know.
And that is only thinking about the impact of tech we might see this century.
Exponential growth is really a phenomenon of transitions. We are in a very rapid 10.000 year transition from biological system to cultural system. This transition accelerated further in the last two centuries. If we think about the end point of this transition it should be a system that is entirely defined by its culture rather than by its biology. Maybe even completely away from its biology. It's fun but idle to speculate what that culture will be like, but if experiences can be shared as easily as text today, what is the value of having billions of copies of similar experiences? What _is_ an individual in that situation?
Even if we accept your simple expansion/competition/ressources based model of reality* (and I see no reason we should) maybe combining consciousnesses leads to much better results. Maybe expansion will look like ever increasing use of energy resources to enable larger consciousness rather than maintaining more animal bodies.
Population is projected to plateau in the near future. What if those habitats exist but just are not observable since they are quite small compared to their stars / planets?