Originally Posted by MarkOK

There are 52 Business class seats on a 77W. There are only 8 F seats.
The provides space for the systems to double upgrade from Y->J->F because a Y passenger has a decent chance to get into J and then apply the second instrument to get into F.

There are 28 PE seats. That is a bottleneck towards upgrading into 52 business class seats. It'd be quite a different maneuver to go from Y, hope to upgrade to very limited PE cabin, and then hope to upgrade to J.