But still, surely they can scale / upgrade their existing infrastructure, without having to go through the process of putting up a new tower...?
Congestion isn't about just upgrading.
The current equipment can actually handle quite a bit but spectrum is a license limitation.
Think of this very simple explanation. You have a pipe that can fill a bottle of water in 20 seconds. You have a license that only allows you to fill it at 10 seconds. You can do it at 20 seconds but not allowed because that other portion of the pipe belongs to another network. SO what do you do? You add more pipes and that increases the the water flow.
Same with spectrum. There is physical speed limitations directly linked to the spectrum they own.
I also remember you were told that 2600MHz is not available on the tower. Rain has 2 types of setups. One of them is where they have their own antennas and run both LTE Bands 3 and 38. These towers offer the most bandwidth as it has 4x4 mimo and carrier aggregation setups. Second setup is where Vodacom rolls out the Rain tower. In some cases where I have noticed is that Vodacom shares sector antennas with Rain. This means that those sector antennas do not have enough plugs to run the full Rain and Vodacom LTE frequencies. In this case if Vodacom can spare a couple of plugs on their sector antennas they only run LTE Band 3 in a 2x2 mimo configuration and house the equipment in their cabinet.
2x2 mimo on LTE band 3 with only 10MHz spectrum is sadly a very slow setup with a max speed of around 100Mbps if you are lucky shared. That means Vodacom customers roaming, Rain fixed and Mobile users. It's a sad affair of congestion but it is what it is.
The upgrade would be Rain going out to the tower. Installing their own sector antennas and run the 4x4 mimo with Band 3 and 38. It offers the most bandwidth. After that it would mean more towers.