I’ve noticed some FOs seem to not get this.

They have only seen their FO seniority improve, saying things like “I have xxx people below me” as they benefit from FOs upgrading and pilots being hired below them. What some fail to see is once you upgrade to a narrowbody plane, at a narrowbody airline, that’s it. There is no more movement short of retirements. Nobody is leaving off that CA list ahead of you, short of retire or die. The widebody airline creates that flow from the CA Narrowbody side as well, it’s incredibly beneficial.