I agree that it would make sense that J>I. This could be bad for all of us using SWU and SSWU on a regular basis.
However, I have been on a few United flights lately where this option was offered (for example, Upgrade to Economy Plus for $29 or Upgrade to First for $149). These were on US doestic flights where the UA J cabin was bigger than 2 rows. The J cabin did not completely fill up on these flights where this was offered.