"There is a high degree of uncertainty," Ovum's Mike Roberts correctly notes in his estimate, but his 24M projection is consistent with other datapoints. (My wild guess would be lower.) MU-MIMO and Massive MIMO - in sub 6 GHz frequencies - will almost certainly be more important into the next decade.
High frequency millimeter wave will be an important part of 5G many places one day and working on rules is sensible. The hype, from the U.S. FCC and the vendors, is so inaccurate it's almost funny.
The most aggressive promoters of millimeter wave - Verizon, NTT, Korea Telecom - have been clear: trials only until 2020-2024. VZ CFO Fran Shammo made a point of telling Wall Street that the actual deployment will be so small it won't affect capital spending for years. NTT CTO Seizo Onoe in 2015 said he expected his 5G deployments mostly will be below 6 GHz until 2022-2023 and confirmed that this year. Giant China Mobile just joined Qualcomm in a demonstration of 5G in sub-6GHz spectrum at Mobile World Congress. (Release below.)