I think its ok if low level monks can neutralize single big hits, and if high level monks are decreasing incoming hits at a lesser degree. This forces higher level monks to explore their new bonus actions a bit more -- especially starting at 11th level, the monk is moving from thinking about deflecting and transitions to proactively moving with impunity around the battlefield to a variety of different effects.
Now, if Deflect Attacks did scale better, it'd probably be best if it scaled so that when you use the feature, you deflect 1 or 2 additional attacks before the beginning of your next turn. However, by the time you would need scaling, your martial arts die is already d10 or potentially d12. So, you'd be making it weaker earlier on, and buffing it way more than it already is from mid level and onward. So, I think it's best for deflect attacks to stay as it is, and to just graduate to different damage types higher up.
Alternatively, it'd be cool if they gained another potential reaction they could use, like to use Step of the Wind or Patient Defense as a reaction. Or maybe even allow the monk to spend a DP at the end of their turn to have two reactions instead of one starting from maybe 10th level or so.