Actually a single delegate would be enough to keep the old chain alive.
This can be mitigated by publishing a 0.9.9 version that stops creating new blocks after the snapshot date, and requiring delegates to upgrade to 0.9.9.
From our experience with the PTS-DPOS upgrade I can say that it is much more important to coordinate the upgrade with all major external entities, like exchanges, block explorers, CMC, ... - if no one is using the old chain anymore then it doesn't matter if someone keeps generating blocks on it.