> If you take a plea deal because you were convinced you'd be prosecuted otherwise, well, that also sucks

You are completely sidestepping the thrust of the grandparent commenter’s comment, which is that the cost of defending yourself from prosecution is prohibitively expensive and punitive in the sense that the outcome is worse than negotiating a plea deal.

> if you took one under duress, then that would be why the higher courts exist, to invalidate your guilty plea when taken under duress.

In this hypothetical the accused doesn’t have the money to pay for a lawyer; they aren’t going to be beating the case on an appeal.