I disagree on several counts. First off, I don't believe Morris is a "better" fit for Dallas' ZB scheme. I think they are on equal footing there.
Secondly, McFadden has never been a productive back with a minimal number of carries like you're suggesting. He performed exceptionally last year in games where he had 15 or more carries.
Third, don't look for a Turbin-type back to replace Dunbar. Dunbar's role, which is very important in Linehan's offense, requires a shifty, quick, fast, "scatback". Turbin (or anyone like hm) is not a fit for the role. Dunbar has his place on the team solidified, unless the Cowboys draft the Ervin kid. He's the only one coming out in the draft that fits that particular physical profile.
As for who gets the starting nod, I think it's truly up in the air. It's McFadden's job to lose right now, but that does not mean that he WON'T lose it