Now this is a really interesting topic!! I often find myself hearing people say "we're gonna evolve to lose the pinky toe" or "we will evolve so no one will have appendixes" but exactlyyyyyy what you're talking about is stopping just that.

Simply put, our medicine is too good and our populations too great to ever see evolution occur again in the human race... or atleast evolution that isn't caused by some insanely drastic change in the global environment or population.

Your definitely right, with modern medicine it allows us to in a form, defy natural selection and allow unfit genetics and lineages to continue to thrive. I think the only evolutionary hope for humans is transhumanism, or "bionic men"... but this isn't a traditional form of evolution as you would probably understand. We could start making ourselves more vital through robotic upgrades, but we would still be the same genetically crappy meat sacks under it all.