The problem is that the Js are poorly made. They were then, and they are now. Back then, musician's were grateful for the tools available and didn't bitch and moan and whine about tolerances in neck pockets that exceeded the thickness of a piece of paper. That was part of why they were junky though, and why there is a higher expectation of J's now in the small company market. A number of other companies took the design and improved it by actually caring about the end result. Treating it not as a unit for sale by a dealer, but rather as a real instrument that will be cherished by the musician who plays it.
I make instruments with a similar construction to Leo's, as I like the strength, ease of fabrication, and tone of the basic blueprint though I've altered the contours to make the traditional bolt-on balance and sound better. Unfortunately, most folks can't afford the Infini, and MDM basses. Also, many of my players (being that I build primarily for pro session and touring players) actually like the "J" body style because they are used to it. They grew up with it.
REALIZATION:
I've had countless J basses on my repair bench over the years and I've noticed something about them which is kind of odd. THEY ALL SUCK. The reason die-hard vintage J owners love their refuse is the same reason they would complain about the cared-for good stuff. The inherent crappiness is what people actually love about these old hunks of garbage.