That very little can economically be built on existing SFZ lots even with relaxed zoning is actually a feature, not a bug, for getting this done. People want change to be slow. At least to begin with, it's better strategically if it takes a couple years and gradual tweaking to make lots of building happen.
The thing that really kills density in most cases is the height restrictions. A lot of the upzoning in my area has resulted in ugly, wall-to-wall low-single-digit floor count buildings with near zero setback. It's better than single family but it isn't particularly dense and it's a huge step backwards aesthetically.
Of course the a vocal minority is fuming about higher density.
It helps that zoning matters more in Oak Park (and Evanston) than almost anywhere else in Chicagoland.