Oliver dying this soon means events won’t unfold the way he initially foresaw, and that it’s entirely possible the Anti-Monitor wins and humanity is destroyed.
Oliver’s death was a shock to all, including The Monitor and Pariah.
The Monitor is ultimately the most shaken, as everything he laid out to that point was a road map for what would ultimately save humanity from the Anti-Monitor.
This was why Pariah dramatically announced “everything is doomed,” before the credits rolled.