The simplest answer why Stewart starting is better for the overall good of the team is rotational balance. Reducing the argument to Lewis starting versus Stewart ignores the rest of the forest. Are Kaleena and Breanna minutes the only concern? Isn't Morgan Tuck likely to deserve significant minutes? Didn't Kiah Stokes show enough to deserve minutes in competitive games?
Start Kaleena in a four guard lineup and you are at a minimum taking 8-10 minutes in playing time away from the post players as Geno is unlikely to sub regularly before the 1st television timeout. Given the talent of the post players this season that doesn't seem like what's best for the team. Breanna will still get her 25-30 minutes, but someone else that is deserving will miss out.
Having 3 quality reserve players coming off the bench for 2 positions in Breanna, Morgan, and Kiah also makes little sense. (A year or two down the road maybe Breanna and Morgan are both on the court at the forwards with Stef or Kiah at center, but it is not a good idea as freshman. Perimeter defense is typically the hardest thing to adapt to when changing levels in basketball. Outside of Kelly Faris freshman guards almost universally struggle defensively even if he they come with a reputation for being a good defender. Asking freshman post players to guard college perimeter players is not a good idea even if they are super talented. Someone as talented as Maya struggled significantly with it when she was forced into it as a freshman. ) By starting Breanna you get those 8-10 minutes back for Morgan and Kiah, and you have two post players backing up two post players.
And by balancing that the rest of the rotation balances itself by providing another more experienced backup at guard for the 3 perimeter positions. Giving the team 2 or 3 backups at those positions instead of 1 or 2 depending on the play of Moriah and Brianna. Kaleena likely continues to fill the 6th man role as Geno prefers it with a guard/wing that is instant offense. Breanna has a huge impact on the game, but she's not really that instant impact scorer that Geno favors. Maya was something of an exception as a freshman 6th man that wasn't a guard, but he had five returning starters and Maya did have a burning desire to score immediately. He could still start Kaleena too, but I suspect Geno feels differently about demoting a senior from the starting lineup than he does promoting a freshman into the starting lineup over a sophomore. That is where he does reward seniority. Starting Breanna provides rotational balance that I don't think Geno will ignore because it will make the team function better.
Well, I totally disagree that Stewart will not be that impact scorer. She is an excellent 3 point shooter, has a great midrange game, can hook with either hand and is a terrific post up player.
And her immediate impact defensively will be something to watch off the bench.
Uconn will have a true 10 player rotation, so I will amend my minutes to say that both Lewis and Stewart will demand 25 minutes. As will Hartley. And of course Tuck and Stokes will deserve minutes. Somewhere in the 15-20 range. There just aren't enough minutes in the game to play everyone 25-30 that deserve it.
If he does start Lewis, Doty, Hartley, Faris and Dolson, there will be multiple subs at the 6-8 minute mark. You're a better analyst of the game if you can say with any certainty how this will all shake out. Stewart, Jefferson, Tuck, Stokes and Banks all deserve minutes. Who is to say he won't just make wholesale changes and replace the starters with those 5 at times. When you have players that can play mutiple positions, I would challenge anyone to predict the rotation.
I will say that the MVP of the BE tourney made her case to replace Hayes in the starting rotation and I can't see how starting Stewart would be more beneficial for the team, even with your fine analysis.
Just so show how versatile this team will be, here are the possible positions for the payers.
Hartley - 1,2
Doty - 1,2
Faris - 1,2,3,4 (I really think she can play all 4 of these positions)
Dolson - 5
Lewis - 2,3,4
Stewart - 3,4,5
Stokes - 4,5
Jefferson - 1,2
Tuck - 3,4
Banks - 1,2
Geno will have a ball mixing and matching.