The submarines from all nations were so utterly useless against that old dragon that the sea monsters were enough to pose as a lethal threat to the underwater crafts.
Only anyone like the big five had enough resources to fit defensive measures on their submarines. Such defensive capabilities came from more than the sturdiness of the alloys they were built of.
Regular submarines no longer even dared to leave port; doing so would be akin to someone walking straight to their death.
Because they were still overwhelmed by the massive number of all of those sea monsters, even the nuclear submarines from the big five had to be very careful when taking them on. Those five only needed to compete and fight against each other in the past, after all.
There was simply no way for all five of them to gather even more than 1000 submarines in total.
Compared to the number of submarines that they had, there were 10000 times as many sea monsters roaming the oceans.