And when you get up a few ranks you’re going to be so bad on everything else that you’re basically going to plateau as Moira and not be able to play anything else without deranking.
The DPS Moira route is a great way to force through some rank ups, but its not a great hero for learning how to play support well. You will be just as good on Moira if you get better Ana, but you won’t pin yourself into a corner where you’re d-gsh-t on anything other than Moira (or pocket Mercy) and throw when you have to play something that requires a bit more skill, gamesense, better positioning and playmaking.