5G might end up cheaper than FTTP, but I wouldn't get my hopes up on it being massively cheaper.

Higher frequencies will require either line of sight or very short distances to the small cell. The small cells themselves will incur costs both CAPEX and OPEX.

Basically the only part 5G will replace in a FTTP network is the drop. And that's where the density and the topography is cooperating. Whereas if you install a fiber drop, you'll be set for 20+ years and you won't have to install, maintain and power a small cell forever.