Piers is free to say that if he believes it, but others are also free to point out he lacks any sort of expertise in the subject matter. Every gymnast who has spoken on this, including notoriously rude Khorkina, has been supportive of Simone actions, and expressed fear of what could have been.
Simone scored the 4th lowest vault score in the whole team final, for someone you would expect to get the top one, that is pretty disastrous, not a simple mistake. She was lost in the air, called the 'twisties', and to correct that can take time, it's a mental disconnect between mind and body, for her to then throw more skills and have it happen again would be downright dangerous, she was lucky she landed on her feet after that vault, she might not be as lucky if she continued. Gymnastics is an inherently dangerous sport, if it goes wrong people get paralysed, people sustain life changing injuries, and sometimes people die.
She didn't let her team down, in fact her continuation could have lost them the silver medal they got if she felt the need to 'prove herself' and made major mistake again. She knew the other 3 women are great all arounders so would be able fill in for her, not quite as high as her usual scores, but they are all very talented. Let's not pretend the rest of the USA team were incapable, that's frankly rude considering they are some of the top gymnasts in the world.
The routine she performed today was changed from her normal one, removing all twisting skills, she has been able to practice this since last week in a safe training environment to ensure she could do them without risking her health - competitions don't have big crash mats and foam pits which give gymnasts soft landings.
Would you really rather Simone have continued in the team competition only to sustain a horrific injury, or further damage USAs score? If either of those things had happened all the headlines would be "Why did Simone keep going" "Warning signs ignored" "Avoidable disaster".
I, for one, am delighted I didn't watch her break her neck on live TV, just so people not at all interested in gymnastics two weeks ago wouldn't call her a quitter.