I think I'm a bit confused by the timelines.
I would have assumed the first cables would be more quickly decommisioned and they apparently last around 25 years. If it wasn't to do with money/geopolitics or literal geography i'd have thought more would exist by now in some places?
It's been a long time since I learned about mass communications, but the cables were really about the American and British wallstreets doing money stuff, and communications was a secondary benefit that convinced people to fund it. And from Britain you can run through Europe and beyond