Most experts thought it would require fiber. Karolina Wikander is confident. "Microwave backhaul technology is already able to handle 100 percent of all radio access sites' capacity needs, both now and in the future. By 2020, the technology will have evolved to support multi-Gb capacities in traditional frequency bands and beyond 10 Gb in the millimeter wave."
Most plans for 5G assume extraordinary reliability. The telcos want to control traffic and connected cars. Failures there - including in massive snow or rainstorms - can cost lives. Controlling the cars may require 1 millisecond latency. One expert thought that would be impractical without fiber. (Dan Warren, a good engineer, wonders whether 1 millisecond is practical no matter the technology.) Update - Ericsson pointed out that microwave latency is low. See below.
Millimeter wave high frequency for the 5G network itself is deeply troubling for policy.