I have only been a ham for 4 years so I don't know how it was years ago.
Some of the smaller ones may be declining but the larger ones are going strong.
These seem to be doing fine:
Ham-com in Plano Texas,
Belton Hamfest in Belton, Texas
Ardmore OK Hamarama, is actually growing. This year they have rented more space in the Ardmore Convention Center.
My only complaint about hamfests is that they shouldn't let vendor sell stuff that's not ham related - except for food of course.
A hamfest is not just about the flea market, it's meeting up with your old friends and making new ones.
I will however admit that if I had any newer gear to sell I would put it on Ebay as I have a larger audience.