He has a couple options
1 - Try to resolve the issue with his back so that he can train and compete at an elite level.
2 - Retire from competition and move on to the next phase of his life.
If he retires, there is no doubt that Nike will take care of him. He is one of the most decorated runners in American history. Nike will sign him to a contract/ legends endorsement deal (a la Joanie and Alberto). I am sure they would let him coach or work for the company if Galen wanted to. But Galen has probably made many millions running over the course of an 18 year career as their top-paid American distance runner that he can elect to sit back and focus on being a parent for a few years.
Also, while Galen secluded himself and does not appear as openly as Meb did in his prime years, Galen is still a huge name. If he does a photo/autograph signing at a major marathon expo, the line will be around the hall to see him. Because he’s Galen Rupp.
It is possible he can recover from the back issue. The only question is whether there will be ongoing nerve compression preventing him from training 100%. He could elect to have surgery, but once you start cutting, he will ultimately have these same problems re-appear down the road. If he lets things heal naturally (which he should try to do), there’s no telling what the final outcome will be on ability to perform at an elite level or how long it will take to get back there.
Nevertheless, I wish him the best. I’ve always loved and respected Meb because you know if he toed the line he was going to leave it all out there. But I feel like with Galen, when he toed the line, it felt like he was going to do something really special. I mean - he sat at the starting line in Atlanta patiently waiting for the race start, knowing he was going to destroy everyone in that field. He was so far and ahead better than everyone else it’s not even comparable.