People seem to not understand that a peak player is not the real player. Mary was able to play at an incredibly high level from time to time, but she wasn't able to sustain that level for long.
Serena, Graf, Navratilova or Evert are on everyone's list as the best (in different order) because they were able to sustain a high level tennis for a long time, and even more, they were able to win big tournaments even without being at her top level or in their best form.
1994 is a good example why Pierce not achieve more. She was in top form, she destroy all her rivals, including Steffi Graf in semis, but she wasn't able to be that good in the final. Arantxa simply doesn't let her play the way she wanted, she had to hit one more ball in every game, and finally she started to make more errors. Her finals against Henin and Clijsters are good examples of that too.
In a good day Mary could defeat everyone, but when she wasn't in a good day, the number of players who were able to defeat her were really high.
And mentally she was very far from the ones that dominated the 90's
Pierce was as good to beat anyone at her best but that level she was not consistent or maybe she was still beatable afterall.
Its actually a myth that Pierce was all injured apart from 1994 French Open, 1995 Australian Open, 97 Australian Open,2000 Roland Garros and her 2005 slam run. She played lots of grand slam events and she was just good enough to make it past the Qf round just 6 freaking times out of 52 times, about 12 percent of the time. And she should be the best of her generation?
Her slam semis oponnents. (Graf, Martinez, Coetzer, Hingis, Likotseva, Dementieva)
And out of her 52 slams she has another 8 QF rounds to her name on them, not really outstanding numbers in comparisson to the greatest ever, or even players below her in slam count.
Going back to your post i agree completly, a great bunch of players could and indeed beat Pierce in the biggest stage over her carreer, that is why she achieved what she did.