Probes can also be made very fast, so if combat takes any length of time, planets could keep chairing them out
This is a separate issue which will need to addressed, thanks for bringing it up. Planets in combat should have their production queues halted while the planet is in combat (after all, the colonists are busy earning the ATK points per m). It won't stop the dedicated player from flooding ships in from other planets, but at least then it requires dedicated micro-management to pull off.