Interrupts are part of the challenge of a timed M+ run, and the basic challenge is fun. The specific challenge enjoyment for interrupts is going vary between those with a short interrupt CD or multiple interrupts (generally melee DPS & tanks) and those with a long interrupt CD (generally ranged DPS). Unfortunately this kind of drives the composition to always have at least one melee (3 ranged can be done, but it’s not as easy imo).
Given the statements above, I think all specs should have the same CD for interrupts (I don’t care if it’s all 8s CD like melee or an “average” of the 8s and 24s CD for ranged, but don’t bias towards such a large population like “ranged DPS suck at this.”).
Maybe the devs think it all comes out in the wash since there have been boss fight designs that require melee to run out (0 DPS) while ranged are just plugging away (yes, I know, about throughput, not specifically interrupts, but I think the devs consider balance across all aspects of an encounter not balance of individual aspects in isolation).