Indeed rural areas are not as well covered [1], but in cities it doesn't matter if you live in an old building. As long as your street has fiber (most do by now), when you switch to a fiber connection from your ISP, contractors will connect it to your house (even if you only rent an appartment) and install an optical outlet in your appartment, all for free.
[1] https://map.geo.admin.ch/#/map?lang=en¢er=2634455.81,119...