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The church was already built on the apostles and prophets at that point in time, but the roles did not cease in the church. Revelation talked about prophets in a future date (at least from the author's standpoint, no matter which of the major conservative views you take.) Plus, from a local church standpoint, Paul was involved in the foundation of individual churches before and after he wrote this verse, in his churchplanting ministry.

After the foundation was laid why would there be a need for apostles to continue. Your premise that because Paul was still (at the time of this writing) still involved in church planting mission doesn't mean that more apostles needed to come into existence. Just as Christ taught his disciples and then left so too the Apostles trained elders knowing that they too weren't long for this world but that the people that they trained would carry on for them as overseers and shepherds of the church.

Peter

If you believe what you like in the gospels, and reject what you don't like, it is not the gospel you believe, but yourself. Augustine of Hippo