There's no benefit to duplicating the ~14hour Newcastle to Ijmuiden route
Except there is: it would reduce the additional journey distance and travel time for all passengers in Scotland.
From Edinburgh you can drive or (try to) take the train to North Shields for the ferry to IJmuiden (and taking the train presents all the problems that taking the train to anywhere else in England does). From North Shields, if you go south as far again as you did from Edinburgh, you get to Hull where you can sail to Hoek van Holland. If you go as far again and add maybe another hour, you get to Harwich for the Hoek van Holland ferry.
But somehow anywhere north of Newcastle is out of scope, out of mind. Scotland no longer has any ferries to the continent. It should have a ferry out of Rosyth, and another out of Aberdeen. And if we are being fair about roughly equidistant ferry terminals, perhaps the government could consider building Wick International.
I'm still amazed that you can no longer get a ferry from Shetland to the Faroe Islands, and have to fly. An actual travel itinerary I saw once was ridiculously motorcentric: you were expected take the Lerwick car ferry to Aberdeen, literally drive down to Dover, take the Eurostar to Calais, drive up through Belgium, Germany and Denmark, and take the car ferry from Hirtshals to Tórshavn (which takes 37 hours on its own). Total distance was well over 2000 miles.