You’re right, that’s obviously the motivation for starting to use the higher frequencies. 5G is the vehicle for achieving that, because it hasn’t been done before. But that doesn’t mean the lower frequencies are therefore useless. You said yourself that there is a range or signal strength advantage to using lower frequencies. We still need them, especially in less densely populated areas that can’t have a base station every two inches. Hence, 5G supports lower frequencies as well, and can use them a bit more efficiently than the older generation standards, when the time comes.
I think there are two main bands that 5G is being deployed in today. Some network operators are initially only going for the band that is just above 4G, and some are going for the insanely high frequency band first, or they’re deploying a mixture of both of them. I don’t imagine any network operators are replacing 4G service with 5G yet, but I could be wrong. (I can imagine that might happen in Korea or Japan before it happens in The West.)