Is become a stream-enterer (sotāpanna) enough?

I've quoted below from Ratana Sutta (The Discourse on the Jewels). >Ye ariyasaccāni vibhāvayanti Gambhīrapaññena sudesitāni Kiñcāpi te honti bhusappamattā Na te bhavam atthamam ādiyanti Idam pi sanghe ratanam panītam Etena saccena suvatthi hotu ~ Cited from: Ratana Sutta (Pāli Text) which means; >Those who clearly distinguish the noble truths, which have been well preached by the one with great wisdom, no matter how great they become in heedlessness still they do not take up an eighth existence— this excellent treasure is in the Sangha: by virtue of this truth may there be safety! ~ Cited from: Ratana Sutta | The Discourse on the Treasures According to the above, stream-enterer will not have an eighth existence (re-appearing) no matter how great he/she become in heedlessness. Does it mean that a stream-enterer will become an Arahant within his/her next seven existences even without any effort? If so, why would someone put an extra effort to become an Arahant after becoming a stream-enterer?