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.
Yeah I used to live in Zurich before moving into the boonies. We were the first party in the house of 8 to want fiber, filled out a form, ISP contacted house owner and everything was up and running in a manner of days.
I now own (or rather the bank does) a small house in a more rural canton, and while it’s possible to apply for fiber, you have to pay a significant part of the cost yourself, last I checked it was more than 5K CHF. Not really worth it. I get 400 MBit/s over copper at home from Init7. When fiber is rolled out in the street, they will upgrade it for free.
[1] https://map.geo.admin.ch/#/map?lang=en¢er=2634455.81,119...