Being a WR coach under DeBoer is a highly coveted spot. It's a resume booster, like being a LB coach under Saban or Smart. Shepard was never going to be a long term stay, and bot sides knew that... he's be close to leaving before for promotions. His predecessor under DeBoer, Kirby Moore, will be a HC sooner than later as well... he's just still a tad young for many jobs. But DeBoer will have no issues with a shortage of candidates if he chooses to go outside the staff and replace Shephard.
And his Linguist pull from Buffalo 2 years ago was so impressive at the time (and still is) that I have zero concerns about him replacing a guy or 2 on defense either... though I really hope (selfishly) that Linguist sticks around a bit longer. But again, it's more of a rental situation with him too.... sooner or later he's going to take another job.