To your point about fewer kids saving us money -- that's been brought up before. It's really confusing! But basically there are three things going on.
The first is simple: what families bring to a community can't be measured in monetary terms only. Families also bring stability, creativity, safety, love, hope, and joy. In the same way pets, green space, and the arts contribute to a community, a neighborhood without children is a less joyful place.
The second is more complex: You're going to pay for public education one way or the other, and you may as well pay for it to be really good instead of really crappy. I think the easiest comparison is to group health insurance. I didn't want to use this example in my article because it makes it seem like I'm attaching some sort of personal or moral value to being young, healthy, well-off, a native speaker, or without disabilities. I'm not. But there is a difference in the amount it costs to insure some people, just like there's a difference in the cost to educate some people. At my work, we all buy into a group health insurance plan. Because there are a wide variety of participants, some of us cost almost nothing to insure, and others (like myself) cost quite a bit. But because the costs are averaged out across the entire group, we all get a relatively low rate. That's kind of how it looks in the school budgets. A school with a wide range of student need functions better socially, academically, and financially. You are going to need pretty much the same number of teachers, ESL people, therapists, occupational therapists, gym teachers, etc. You still need to pay for the building maintenance, heat, lights, water, snow removal, etc. But if the school is underenrolled, with a high special-needs population, you're paying out a lot of money for a handful of kids. If you have a bunch of typically developing kids mixed in, you have a full school with full funding, and the extra stuff you brought in for the special needs kids can be available to everyone -- for example one of the ESL teachers at our last school was also the librarian.
If you don't fill the schools, eventually they close. A closed school is traumatic for kids. It's traumatic for parents. It's traumatic for a community. It shows that the community is shrinking, not growing. It shows other families that this isn't a good place to live. And you still end up paying -- this time for transportation. Because if you close the school kids can walk to -- and the majority of students in Allston-Brighton are now in a neighborhood school -- you have to pay for them to ride the bus to a school farther away. In addition to the high cost of busing, it means that the family doesn't engage in the community to the same degree -- they don't have time. They also don't engage with the school -- it's too far away. Parent participation is one of the key determinants of success in kids -- if a parent can't get to the school, they can't participate in their child's education. So again, there are monetary costs and also non-monetary costs.