> Until whatsapp dies. Subway systems run for decades. Can anyone really trust that any particular app will still be around in twenty years? Or two? For critical infrastructure you need something stable, something like a part of the RF spectrum dedicated to critical infrastructure. This is why cops/pilots/ships still have dedicated radios even though some combination of cell/satellite phones would certainly be much cheaper in the short term.

The radios of those are outside. The radios of subway are underground. It's not great for radios, requires infrastructure, and phones solve it in same way, by having infrastructure close to cover for reduced range.