One important question that I'm unclear on is how long it takes to fix one of these cables. If it takes months then that is quite a wide window in which an attacker could incrementally take down cables.
In this particular case, it seems like the attackers were trying for plausible deniability (making it look like an accident with an anchor). A comprehensive series of "accidents" wouldn't fit that goal.
(And if they decide they don't care about plausible deniability, they could use sub-deployed timed mines to take out every cable at once.)
They could even blow up all cables at once. Maybe the explosives have already been placed.
Update:
> Finland, Sweden complete repairs on Baltic Sea cables
Generally it can be fixed in days. They raise it from the sea floor and splice in a new cable section.
Certainly worth blowing up some russian ships to make sure it doesn't happen again
This is a great video on undersea cables https://www.youtube.com/watch?v=AFt9le2ytW0
"Sabatoge" and repair is discussed at 11:45