For example, look at "bumping rights". If a company needs to eliminate a union position, and this is occupied by someone with say 20 years seniority, that person can "bump" some other union member out of their position who has a lower seniority. So, that person whose role was eliminated can push a person with only 5 years seniority out of their position. And then that person with 5 years seniority can bump a person with only one year seniority out of their position. And the person with 1 year seniority has no one newer than them so they get laid off.

Was it in the best interest of that newish employee to be part of a union? So they can act as a meat shield for someone much further in their career who would theoretically be much more employable in the general market?

They won't frame what they consider to be their self interest as naked self interest though, they'll dress it up as concern for the average worker or an opinion that organizing is ultimately futile because sometimes you lose.

Definitionally unions exist to preserve the incumbents.