Biles' 2016 Olympic performance was far more impressive than Retton's 1984 Olympics (even if you ignore the fact that many top competitors were absent in 1984 due to the boycott.)
Biles' non-Olympic career is also much more impressive than Retton's.
In the vault she did the other day, she lost her positional awareness, and then was unable to overcome that in practice. On the vault, they land on that huge, soft mat and she managed to land on her feet. In other events, they land on a thin, firm mat. If that happens in those events, it could easily result in serious injury.
She could probably get over this, given some time. She doesn't have that right now. If she doesn't get over it, she's done as a gymnast.
Retton was the first woman to win the all around gold medal. She was and still is hugely popular. She was also a big supporter of Ronald Reagan. She set the table for Biles by inspiring a generation of girls to take up gymnastics. 10 years from now Biles will be a name in a record book and not much else.