Ephesians 4:11 (King James Version): "And he gave some, apostles; and some, prophets; and some, evangelists; and some, pastors and teachers"
Question: Since all offices were given for the same reason according to verse 12, how do we decide which offices still remain?
By the qualifications for the offices.
Apostles were selected by God as eye-witnesses to Jesus Christ (I John 1:1-4; Acts 1:21-22). Now that it has been 2,000 years since Christ was here, there can no longer be living eye-witnesses. But that is alright because the apostles left us written records, so in this way, the apostles are still with us (II Peter 1:12-15).
Prophets have to cease because the power to speak the words of God directly were passed on by the laying on of the apostles' hands (Acts 8:14-17), but the apostles are no longer physically with us, so prophets too would die out. Besides I Corinthians 13:8-10 says it was going to go away. So does Zechariah 13:1-4 by the way.
The remaining offices remain because people can continue to qualify for their duties.