Here's another fun question to pile on to that. If that end goes in motion 5+ yards from the LOS, is he still a lineman, or a back?
Then once you think through that, you'll see that we need to delete that rule. lol
If he's 5+ yards behind the LOS, he's a back. 2-32-9 defines the positioning of a lineman "when the ball is snapped", so if he's not on the line at the snap, he's not a lineman. You can have a player in "no man's land" who doesn't fulfill the definition of a back either, but not if he's 5+ yards off the line.
I kind of get your point with the phrase "establishing as a back" in the rule, since that's not otherwise defined. If the player never established as a back, is he not still a lineman, after all? The implication/intent of the rule is about where was the player when the motion started. A player starting from the line and a player starting as a back have different rules, but the player on the line can move to a back, stop and then restart as a back and switch rule coverage. It's probably another example of some language that needs cleaned up, but I don't think deleting the rule is necessary -- outside of who has ever called that or paid attention to it?
It's a relatively uncommon shift (an 8th lineman going in motion) in today's game that we'd probably all miss it and nobody would notice. If there were 4 backs prior to one of the seven lineman making that shift, we'd all flag the illegal formation for having five backs at the snap.
