Tosfos asks: Since in principle the kohen she’avar should be treated like a kohen gadol (if not for the factor of eivah), why do we need the sevara of ma’alin bakodesh to tell us that he cannot serve as a kohen hedyot? He's a kohen gadol, not a kohen hedyot, unless proven otherwise?
Tosfos answers that a kohen gadol can be fired from his job by beis din and his fellow kohanim (see Tos Yoma 12b). There is no guarantee that it’s a lifetime tenured appointment. Therefore, if not for the sevara of ma’alin bakodesh, we would have assumed that a kohen she’avar is someone whose kohen gadol is revoked, and therefore can do avodah wearing bigdei kohen hedyot. Kah mashma lan that because of ma’alin bakodesh, even if the status of kohen gadol is revoked, this kohen she'avar is not allowed to go back to wearing bigdei kohen hedyot.
The picture that emerges is a little strange, no? There is no problem of ma’alin bakodesh v’ain moridin in knocking someone who has kohen gadol status down to the role of kohen hedyot, but there is a problem of ma’alin bakodesh for that same person to wear bigdei kohen hedyot in place of bigdei kohen gadol once his status has been revoked.