no backup connecting whole continents
Wtf are you talking about, there are dozens of them, its literally a huge network of cables.
Theres a whole website about it
Also, global economy is highly fragile, it can take one major investment bubble like in 2008 or event like Covid in 2020 to shake its foundations. Countries all over the world have to pump shitloads of money into it to keep the economy in a state where it won‘t collapse.