I think the answer here has to do with the complexities and subtleties of verb tenses which goes beyond the basics of grammar. Here is a simple classification of tense appropriate for both English and Greek. I will be only discussing the indicative mood.
I do not intend to discuss this in detail except to note that verb aspect usually includes the "simple", "continuous", "perfect", "imperfect", "habitual", etc.
Let us define the simple tense as a description of the action from speakers current point in time. Then we have (for the verb "to run"): I ran (simple past); I run (simple present); I will run (simple future). To this may be added for the continuous: I was running; I am running; I will be running. etc
Compound tense describes an action when the speak places themselves in the past or future. Thus we might say: "I was to have been running", "I was to have run", or in the future tense: "I will have run", "I will have still to run", etc.
These compound tense ideas in both Greek and English require either auxiliary verbs or auxiliary phrases. Here are some examples:
- By this time next week I will have already run the race.
- This time last week I was to run the following day.
Note that in each of these the speaker jumps forward and looks back or jumps back and looks forward. Now a famous example from the NT
- Matt 16:19 - ἔσται δεδεμένον ἐν τοῖς οὐρανοῖς = shall have been bound in heaven
The simple narrative in John 1:6-8 says this:
There came a man who was sent from God. His name was John. He came as
a witness to testify about the Light, so that through him everyone
might believe. He himself was not the Light, but he came to testify
about the Light.
Note that John is writing from the point of view of someone recounting the events just after they occurred - more specifically at the time of John's ministry. He recounts the fact that John was sent [past completed action] so that through him people might believe [a then future event].
The same is true of V15 - John the Baptist is speaking in the present [ie, John the Disciple is placing himself in the narrative in the past recording time].
Thus, in the narrative's view point, the events described had not yet concluded.