You are a key target for the airlines - accrue miles at any cost.
I flew to Iceland a few years ago when the cost on Icelandair was so cheap it would have made no sense to fly any other airline, even if there were a choice. And as posted above, other airlines will probably be so much cheaper than Delta, I can't imagine using Delta just to "accrue miles one way or another."
Iceland Air is cheap indeed, but then there is Iceland Express too.
Iceland Express is so cheap for trips that cover Iceland-US that I cannot see how DL can win a major chunk of the back of the flying bus customer marketshare based on advertised price alone.
... and connecting onto DL at JFK is something I avoid as much as possible.