First a technical comment. You're graph descriptions are confusing and partly unexplained (what's WAR-low and WAR-high). You're y-axis says percentage, but it seems to be a ratio (a mitigation of 0.35% vs. 0.20% would be irrelevant). However, some of these ratios go over 1. More than 100% reduction?
The more general problem with all this numbercrunching I see on the forum is that no one explains basic game mechanics. Don't armor and resistances also provide mitigation? Is the PLDs mitigation additive or multiplicative? Is all damage mitigated?

My theoretical complaint with this kind of reasoning is that looks only a part of the role a WAR or a PLD has. Because contrary of what you claim, a tank's job is not to survive and keep others alive.
A tank is part of a group and only the group as a whole can fulfill a purpose. And a group's purpose in any MMO I know of is do damage. Damage is all what counts because only damage turns bosses into loot and progress. Other things like tanks and healers are only secundary functions in order to keep the group able to do damage.
Thus, any claim that it is not a tank's role to do damage is absurd, because it is the only role of any group. Therefore doing damage is integral part of tanking.
Of course, keeping the group alive is more important, because without that damage falls to 0. But if keeping the group alive is achieved, all what matters is damage.
And that's the problem, because contrary to what everybody says, we do not have the means to calculate or reliably measure the difference in damage between a PLD and a WAR. Thus, we can not conclusively compare both jobs.

tl;dr Numbercrunching alone proves nothing