I see where you are coming from but for me the logic doesn’t stack up. Why not 6x3min rounds? Surely it’s in the same vein as Big Circuit tempo work and extensive med ball? Good honest hard work without killing the legs.
BTW I used to like 2-3 x 6 x 30s flat out on the speedball after a few weeks of 3 min rounds but now I find tempos, cals and med ball more than adequate for general endurance work.