Just getting more spectral efficiency when signal strengths are high benefits all users. Lots of towers are at saturation over the air now, so if we can squeeze more bandwidth out of them, traffic that is presently being delayed will do better... even before we account for growth.

That is, 5G isn't just mmwave; it's also getting more out of existing low and mid band spectrum. (And mmwave, of course, will unload the other spectrum too somewhat in the densest areas).