No more confirmation by the coo, the tangle will keep growing, but none of the new transactions will be confirmed.
With the current IRI (1.5.1), the starting point of the random walk is a "not too old" milestone. If there is no more new milestones the random walk will probably became more and more expensive to run. So the selection of transactions to approve (on regular IRI) will slow down. I guess that most instances will start to crash by lack of resources (I expect that the RAM required by the random walk will be the bottleneck).
After reboot, the IRIs will rebuild a new tangle using the latest milestone as a starting point... and crash again...