Its not a argument, its a fact - its easy to play the system to the point where you dont even need to pay a single penny.
Like for an example, this draw I just made just a min ago - costed me only 25 mythrils - you get one mythril for each battle, each day you can get 3-5 mythrils for free just by logging in, and doing special events such as anniversary events, you can get 25-50 per run.
There's no point in paying anything - people who pay are people who have no patience or self control lol.
How long does a run take?
The one where you get 25-50 mythril on special events takes 1 battle - which takes a few seconds because you can one shot the enemy even at a low level.
Regular battles with 1 fight takes me 15-20 seconds (unless its a boss battle which could take me a min or two).
Normal battles (that get you 1 mythril) have 3 fights which usually takes me a min or two.
All of my characters are lv 99 (which is easy to get due to experience eggs you get from special events).
FFRK is really easy and not that grindy.
They also recently added a story mode which involves your original character to go into Final Fantasy worlds and experience the game's plot due to the original story's plot.
I find it a nice time sink when I have nothing else to do when I am outside of my home.
Like I said, your results may vary but I built up my party and cast of characters without spending a single penny on the game.