Northern Light
Superstar
I don' understand why they don't plant a mix saplings with the larger trees. They put in big trees to get a 'head start' but it's stressful and they don't thrive without the amount of care that they clearly aren't getting. A bunch of saplings would get established more easily and quickly overtake the big ones. Look at those birches. A weedy, fast-growing tree like birch shoots up quickly if you plant as a sapling.
I too, agree of mix of sizes/ages is ideal, it reduces risk and expense.
But it also means that the 'result' is delayed.
Landscape Architects and their customers are often impatient.
Young trees are also more apt to need pruning/staking and do have one more issue........they are vulnerable to being shaded out by their larger cousins.
But, on balance, a range of ages, and a range of species types are both moves that help mitigate risk over time.