To send more data per second, you either need a carrier wave of higher frequency (which 5G is doing), or an increased number of simultaneous data streams at the present frequency (which 5G is also doing).
Beamforming can't overstep the physical limitations of a carrier wave, it just adapts the radiation pattern of the antenna array to improve range and reduce interference. This is useful to extend the range of high-band signals, because they operate in object-dense space with a high density of clients. It is also useful at the lower frequencies, because it allows an improvement in spectrum efficiency in an otherwise crowded part of the spectrum.
You are basically saying that beamforming allows more single-user MIMO to improve the data speed of an individual user's connection at the lower frequencies. I agree with that. However, you still need more base stations because (A) you won't see the massive advertised 5G speeds without sub-6GHz and mm-wave, and (B) you need more antennas as you improve MIMO to serve more simultaneous data streams to each individual user at sub-GHz.
I am not familiar with the authors of this paper (https://arxiv.org/pdf/1902.07678.pdf), but it offers a good explanation with some images:
The spectral efficiency of Massive MIMO grows monotonically
with the number of antennas [28]. Thus, we can expect a
future where hundreds or thousands of antennas are used to
serve a set of users. There are, however, practical limits to
how many antennas can be deployed at conventional towers
and rooftop locations, for example, determined by the array
dimensions allowed by the site owner, the weight, and the
wind load. [...] Nevertheless, the spatial multiplexing
capability of these two dimensional planar arrays in our
three-dimensional world is far from what has been demonstrated
in the academic literature, where large one-dimensional arrays
are often considered in a two-dimensional world. In many
practical deployment scenarios, the user channels are mainly
separable in the horizontal domain [35] since the variations
in elevation angle between different users and scattering
objects are relatively small. [...] However, to deploy more
than a few hundred antennas per site and to obtain a truly
massive spatial resolution in the horizontal domain, we need
new antenna deployment strategies.
Instead of gathering all the antennas in a single box,
which will be visible and heavy, the antennas can be
distributed over a substantially larger area and made
invisible by integrating them into existing construction
elements.
Also, you're going to see mid-band (sub-6GHz) rolled out in a lot of places where mm-wave wouldn't be appropriate.