By allowing 7 goals against the Netherlands, Greece no longer controls its own destiny. Even if Greece beats Serbia, qualification isn't guaranteed.
Here are the scenarios:
If Greece beats Serbia: Greece advances as long as Germany doesn't beat the Netherlands.
If Greece draws Serbia: Greece advances if Germany draws or loses to the Netherlands.
If Greece loses to Serbia: Greece still advances if Germany loses to the Netherlands.
If Germany beats the Netherlands: Greece is eliminated regardless of the result against Serbia.
The reason is that if Germany wins and Greece beats Serbia, Greece, Germany and the Netherlands would all finish on 11 points. That creates a three-way head-to-head tiebreaker, where the 7–0 defeat against the Dutch becomes extremely costly.
However, if Germany and the Netherlands draw, Greece holds the head-to-head advantage over Germany, so even a draw against Serbia would be enough.
Bottom line: Greece no longer controls its own destiny. Germany must drop points for Greece to advance.