Even the regime knows the system is rotten, which is why it's afraid
The ruling coalition will certainly not accept an election defeat in the case of a small margin, which is possible with average turnout. If seventy percent or more of voters went to the polls, that margin could be convincing. Even that is not a guarantee that Vučić will accept the results, but his room for maneuver will be smaller.

TL;DR
- The ruling coalition is unlikely to accept an election defeat with a small margin.
- Average voter turnout could lead to a close election outcome.
- If 70% or more of voters participate, the margin of victory could be substantial.
- High turnout may reduce Vučić's room for maneuver but doesn't guarantee acceptance of results.