This isn't intended as a personal attack, as we are all trying to provide advice based on our own past experiences, both within and outside of BA.
I've been on the receiving end of the "it's not so bad being junior, we've all been there, you only have to be junior once, it gets better" chat from those I fly with. You then probe a bit deeper and out come stories of Bid Line, Clash and Protect, 600 hours per year and rapid movement up the seniority list (in one instance caused by c. 300 BMI pilots being added to the bottom of the list - let's leave those worms firmly in the can!)
We're the first pilots in what, 20/30 years? to be junior under a new bidding system. We're working under a new ruleset (EASA FTL) that is generally regarded as having much less fatigue protection than the old CAP FTL. Many on P32L have lost seniority, some significantly, as CRS pilots were pushed / parachuted onto the airbus fleet (I've not yet met one of them who wanted to be on SH). We've seen what career progression we might have expected (LH, Left Seat Airbus) pushed further away by those same ex CRS pilots who are (and I'm happy to be corrected) unfrozen for any other fleet they wish to move to in the next PRIAM bid. And we have a union that has (in practice, even if not intentionally) abandoned junior pilots as collateral damage).
I struggle to think of a reason to stay other than blind hope that it has to get better as other contributors have said it does, and the thought that after nearly 5 years of promising my family "it gets better, its a long term game at BA" it would be a hard sell to them to move. Again.