What are the practical limitations?

I noticed that the disc diameter is 6'. This would suggest that the maximum depth of trench is likely less than 3' (ie, radius of disk, minus limits imposed by operating mechanism, plus depth increase due to length of bits). The frostline where I'm located (Canada, near Ottawa) is about 3.5'. To safely ensure that I never have frozen water lines, I'd need a depth of 4' (the depth that all water lines must be run in my area), which would suggest that I would require a 9'-10' disk. I'm thinking that this would be rather impractical.

Was there a consideration of using a chain-mounted bit system more like commercial units use? Why, other than simplicity, was a disc chosen as the trench blade?