Given a choice, executives in the mobile business would prefer to use lower-frequency spectrum than high-frequency assets. Lower frequencies propagate better, so are easier to manage when coverage is really important. But executives do not have unlimited choice.
Lower-frequency assets are largely allocated to various other users and even when low-frequency assets can be reclaimed, bandwidth necessarily is limited, as frequency and capacity are directly related. As optical fiber inherently supports higher bandwidth, so higher wireless frequencies support higher capacity.
That means teraHertz communicaitons are going to be part of the future mix of mobile communications, even though we have only begun to use millimeter wave frequencies.
Ultra-dense optical fiber networks will be necessary, as teraHertz signals dissipate after a score or two meters, using currently-available radio technology. So coverage will likely continue to be dominated by low-band and mid-band frequencies, with millimeter increasingly used for high-demand areas where optical fiber backhaul is prevalent.
Commercial teraHertz coverage might well resemble Wi-Fi distances, if even more challenging as signal propagation is line of sight. That suggests optical fiber density very close to fiber-to-home deployments.