It's not that easy as just giving the chance to Henderson imo. de Gea for all his flaws, still make saves that not many other gk's can do. Not to mention that not long ago he was our best player over several years.
Is it unimaginable that DDG gets back to his best form? I wouldn't rule it out. He must also hold some status in the dressing room. If he was suddently dropped from the first team, and Henderson takes over, it could become a bit of an issue in the squad i imagine.
Of course, at the end of the season the board and Ole can make a decision on who to bet on, or if none of them a new player from somewhere else. Doing it during a season is a bit more difficult i feel.
Finding a new club for DDG would be an issue as well, but shouldn't be part of the decision of course.