This is the same arguments that there were in England when the spinning and weaving machines were introduced, when steam overtook sail and when the cars replaced horses.
Work was developed in other industries, or in services etc. There were a lot less people then, but still, there is no need to believe that there will be any difference this time,
In any case, that one little ship starts sailing autonomous between two ports a few miles apart on the Norwegian coast in 2020 doesn't threaten the job prospect for Norwegian seafarers immediately.
Much less for US mariners. With the oldest fleet in the world and a total inertia against development, it will take a long time before that happen. They will go on building ITB units and modified 1970's OSVs for the foreseeable future.
As to navigation being an art that is not so these days, unless you mean switching on the EADIS machine is an art.
It was in my young days, sailing in between reefs in areas where some were charted wrongly by several miles, with magnetic compass, a worn out sextant and no working radar, but with a promise to get a new radar "when a good second hand one could be found".
Learning to detect changes in motions from backwash, indicating a reef, island or coast near by, then listen for breakers as you got closer, is a lost art today, when most navigators have their head stuck against some screen or another. (or checking Facebook)
How to get job security in the brilliant new future in the Marine Industry??
1) Make sure you are up to date on skills that is required at all times by attending any courses that is offered, or by self-study.
2) Be able and willing to change with the times and opportunities that is offered, or create your own. If that take you to a shore job eventually, so be it. (You may get to appreciate it as you get older)
3) Don't listen to old farts that is unable or unwilling to change. They will fail to stop development, just like the generations before them that tried and failed to do so. (Read history)
BTW; Norway do have a coastline and the designers are not nerds. Many of them have been at sea, or have grown up with ships and boats. "The Father of modern Offshore vessels", Sigmund Borgundvaag was a fisherman in his young days. (See the UT 704 thread)