It is important to clarify what we mean by "obligated / not optional", as I think there is a terminology mismatch.

When I said that a particular job task is not optional I mean that not doing this task will lead to a disciplinary action from the employer (being fired, put on a performance improvement clock with the HR, etc.). Reducing those tasks brings in one set of considerations.

Your definition of "obliged", if I understand it correctly, is primarily a self-assigned or a community-expected one: "if I stop running this seminar or remove myself from that editorial board, I will feel I am not doing all I can / my colleagues will look at me askance". But it will not trigger retaliation from the employer. Reducing overload from those tasks brings a completely different set of considerations.