There are plenty of good Freshmen in the starting lineup of Top 25 teams, so I don’t think she “needs to be” coming off the bench.
Donovan is an undersized post player who doesn’t shoot threes and isn’t nearly as productive as, say, a Liatu King for example.
I’ve watched plenty of Duke games and I’d say they look a lot better with Fournier (or Wood) to complement Delaney Thomas.
There are plenty of good freshmen starting, but Toby doesn't need to be one of them. There's an even better freshman forward than her who has started ONE game this season (you can figure out who that is). There's a freshman guard in this conference who hasn't started a game and is considered to be just as good as, if not better than, Toby. In fact, she's won ACC Freshman of the Week more than Toby has.
Starting as a freshman means very little in the grand scheme of things. A'ja Wilson started ONE game as a freshman. Breanna Stewart started 13. Why anyone would care about a freshman starting from Day 1 is beyond me. And no, I'm not saying that Toby is the next A'ja or Stewie (she's not that good and isn't going to be that good). I'm just providing perspective.
Jadyn Donovan plays great defense (which is a must if you want to play for Coach Kara). Toby does not play great defense. Thus, one starts and one comes off the bench. [Sophomore] Jadyn isn't [grad student] Liatu King--yet. Doesn't mean she can't become that. But she has provided consistency in what she brings--toughness and defense.
What Duke needs alongside Delaney is consistency (since she's the model of consistency). Donovan is that. Fournier is not. It's not hard to see why one starts and the other does not. The "start Toby" crowd is awfully loud after a game like Virginia Tech or Cal but go radio silent after she no-shows against South Carolina (while her freshman forward counterpart for SC was all over the court making play after play), forces the issue against UVA (getting blocked repeatedly until she figures out that there are other tall people on the court), or plays timid against UNC.