I think it largely depends on whether you are talking about academics or private practice.
In academics, it is quite possible to function solely in the intensive care unit when you have service obligations. We have board certified pulm/cc attendings that ONLY attend in the ICU when they are on service. The rest of the time they pursue their academic careers (healthy policy, epi, clinical trials, basic science, administration). Some of them also attend on our procedure service. Others attend in the unit, on pulm consults, and have clinic. That sort of pulm/cc doctor is decreasing at our program.
Community or private ICU medicine is an entirely different reality and lifestyle. The Leapfrog group's recommendations that all ICU's be staffed by intensivists should push many community hospitals to "close" their ICUs, which in return should provide more opportunities for people mainly interested in critical care. The reality of community ICU medicine is that you are often also the pulm consult and have to go see patients on the floor. I've been told that intensivist jobs are moving to a shift-work based system similar to the ED. Because you don't have the capability to dabble in teaching, research, etc - just being in the ICU week after week might grow tiring.
I think that you can solely do critical care medicine if you want to. I think that there'll be more opportunities to do so in the future.