It's possible to have a node that reports itself as up, continues heartbeating, and thus can keep the primary from stepping down, even if that node is unable to replicate new ops. If a secondary node is unable to replicate ops, whether because of an issue local to the secondary or because of an issue on the primary that prevents the secondary from getting oplog entries off it, the secondary should not in any way keep the primary from stepping down, or prevent an election of a new primary.