My recollection is that one reason given for not increasing the system to J-6 was that the average jump range wouldn't go up enough to justify the increased cost. Personally I think that is just the official line for public consumption and the real reasons are a desire to retain the governmental/naval and megacorporate speed advantage, and also resistance from vested interests - a lot of worlds along the X-boat routes would wither if they were moved, which they would be if a J-6 system were set up, even if the J-6 system only covered the main trunks, leaving the side branches at J-4.

Yeah, look at what happened, for example, to Route 66 as the Interstate system came on line, ( repeated all along the Interstates where whether a town lived or died depended on where the Interstate exchanges went. ) Lots of ghost towns along that route. There would have been enormous resistance to disrupting the status quo.

Also, building the original XBoat routes at J4 must have been rife with speculation and chicanery, much like the building out of the rail system in the US during the 1800’s.