If we can put aside the Dak vs. Romo ability/talent part of this debate, remember the 2016 Cowboys started 0-1 and then won 11 games in a row with Dak. Just about any (competent) coaching staff would want to do nothing to upset the chemistry of the team at that point. It wasn't worth the risk.
Even if Dak graciously stepped aside and let Romo return to the helm, remember Romo would have had to knock off three months of rust (and more, considering he missed so much of 2015). That would have led to some losses. And then the locker room would start to divide into pro-Dak and pro-Romo factions, and the season would have fell apart long before the playoffs anyway.
Just my take on the subject.