How would changing PLD and RNG enmity make the game less challenging? By making those jobs not have to level NIN, THF or MNK to be useful? Its about job balance which typically takes a jobs weaknesses and adjusts them to make them more useful.
I'm not sure adjusting enmity caps for different jobs would do that but its not the most ridiculous idea. Certainly less ridiculous than allowing MNK's 4k HP and WHM's unlimited MP.