I thought the same thing! My mom used it when I was younger, and I always passed on trying it because of that random memory and association.
I just got past the Metamucil to plain psyllium husk. I felt the plain version was going to taste bad, but it was just plain and not sugary. It's funny the concepts that can lead to certain small behaviors.
Contempt without prior investigation and denial of my mortality is the true reason.
But once I started taking it I was sorely disappointed in myself that I did not start at least 10 years sooner.
Perhaps un-branded supplemental fiber is unburdened by the old people connotation? I probably still would have resisted.