"Are humans coming to a stagnation in evolution?"
"tl;dr Medicine makes bad people with bad genes reproduce, stopping evolution."
"...but I also have a nagging feeling that there's something wrong with what he's saying."
One issue with this argument seems to be an assumption that there is a proper line of evolution. Another assumption that medicine is not a form of natural selection. Also, certain mutations could give rise to beneficial functions later on.
Particular genes that seem "bad" in our present time might actually give rise to entirely new functions given enough time to continue mutating. Suppose within a segment of DNA there is a repeat of an entire region of another chromosome. This segment might cause debilitating effects at present, but could mutate with enough recombinations and base pair mistakes to give something that seems like a "good" gene in a few 100,000 years.