In 5 man mythic where you are the only healer and u need to save ppl from dying contrition has a place but in raids you are not alone. How many defensive penances are you going to cast to make a good use of contrition? Your paladin is also spot healing remember?
Disc healing comes in bursts. You prepare for a big burst with atonments and you use your dps canon. In legion we had the artifact, in BfA we have schism/solace/penance. Its almost the same burst but spread over a schism cast (solace/penance can be cast on the move and they land in about 3 sec depending on the distance of the boss. Remember light's wrath had an insane long travel time!!!) You dont need SotM dmg increase buff during that wide aoe healing. You dont care at all! Schism will do the job for it.
The next phase is conservation. This is the phase where sotm is working for you! You have 1-4 atonments running and a perma 4% ++ dmg buff that is a healing buff on those targets. Still you dont need to cast defensive penance. Smite/offensive penance is more than enough when you have 4-10% increase dmg buff from sotm.