It's all about average time per match. That's the best standard to use. If the average time of a match in SFV is less than USF4 and USF4 is already played as 3/5 in top 8s then you have a good case for 3/5 all the way for SFV.
It's not just a case of high damage = more sets. It's about game pace, stun build up, style of play (aggressive vs defensive play), chance of time outs, chip damage, meter gain etc. All of those things factor into shorter round lengths.
And people were lobbying for 3/5 Marvel for a long time before it was actually made into that standard. There was a time where some places were doing 3/5 and others were doing 2/3. The EVO fiasco of pools being 2/3 and then later 3/5 was a total fuck up. Though since then the average match of the game has actually gone longer because when it was first proposed people were in the MVC3 meta where meter build was higher, XF was stronger and zoning wasn't nearly as good as it is now. Still in the end it's just right for the game... 3/5 Marvel ends around the same time as other game's 2/3 sets.