What they should do is make it so that 1-20 the mobs aren't AS hard, and then gradually increase the level of difficulty. This will get new players used to their classes and abilities enough, while also introducing the idea to them which level mobs they need to be attacking. The devs need to remember that new players are NEW to the game, so they don't have everything figured out yet, and if fighting a marmot is incredibly tough just at level 1-20, it could be discouraging.

Difficulty balancing strategy:

Mobs 1-5 = lowered to 20% of current difficulty.
This allows new players to get adjusted to controls and their first spells.

Mobs 6-10 = lowered to 40% of current difficulty.
This will give the new players a change to start thinking how to better use their abilities to best kill the monster, while also keeping themselves alive. The initial basics of strategy.

Mobs 11-20 = Lowered to 60% of current rank difficulty.
This phase will give the new players a chance to start finding groups (with a hopefully refined party search system in the future) and figure out how the XIV party mechanic in this game works.

Mobs 20-35 (when health starts to increase more) = Lowered to 80% of current mob difficulty.
By this time, players will have been able to learn their class, the game, and how to play with others. From this point on, they will start needing to figure out how to better work a party strategy, and start analyzing what types of monsters are weak to what in order to make a quicker kill (Remember, these are NEW players we're talking about).

Mobs 36-50 = 100% of current difficulty.
By this point players should know how to do their jobs and do them well. By the time they hit 50, they should know exactly how this game works, and what they can expect when it comes to challenges in the future.

Obviously my numbers are just percentages and it would certainly be harder to figure out with code, but it's the theory behind it that I think would work if they devs figured out how.