Someone joining now still has many people to come in behind them, but they also have the last 4 or 5 years worth of shorthaul DEPs and FPPs who are eagerly awaiting a move to long haul.
They
may have many people coming in behind, who knows? ... That said you are right to point out that all things being equal and agreements staying "as is" they are behind everybody who has joined before them. In my experience guessing where someone will be in terms of fleet seniority in 4-5 years time is making themselves hostage to fortune..
I also would not be surprised if we start seeing some of the more junior short haul captains decide they would prefer to move RHS long haul due to the decline in their quality of life due to JSS.
That could well start happening, and FWIW judging by a conversation on Yammer very recently there is more than one short haul captain considering/discussing going back to RHS short haul due JSS.....