This is key point right here. They should raise threshold limit from 5GB to at least 20GB, then throttle when congested if u go over that.
That would require investing in network upgrades to support the extra bandwidth. It's much easier and more profitable to limit demand through frivolous pricing policies since the result is more revenue, flat operating costs, and the ability to lie to the public about offering higher speeds which are in fact only possible by limiting everyone's usage.