Then what... a Russian sub can just turn the lights out in Western Europe?
Europe would still have their own power generation... It would just improve generation/consumption mismatches. By island hopping you could shorten the buried lengths, to make replacing damaged sections easier.
Happy valley goose bay to the tip of Greenland, up the coast to kulusuk, across to Iceland, then to faroe islands then maybe Shetland, Bergen, Stavanger etc.
Seems like the sort of thing that isn't unreasonable to study.