They do end up saving some money too, though, not only by reducing suicides and accidents, but because platform doors seem to be considered a prerequisite for "one-man operation" (ワンマン運転) on most lines, and that can help reduce staffing costs.
Most Tokyo stations also have high enough ridership that such capital improvements are a lot easier to swallow than they would be for "concrete slab in the countryside" stations.
Luckily, it's the sort of thing they can do incrementally in the places where it makes the most sense; no need to do every line and every station immediately. I imagine they'll probably continue to do it slowly, just as they're slowly adding elevators and other accessibility improvements to even minor stations.
Most Tokyo stations also have high enough ridership that such capital improvements are a lot easier to swallow than they would be for "concrete slab in the countryside" stations.
Luckily, it's the sort of thing they can do incrementally in the places where it makes the most sense; no need to do every line and every station immediately. I imagine they'll probably continue to do it slowly, just as they're slowly adding elevators and other accessibility improvements to even minor stations.