Unless you're constantly streaming 4K cat videos, most people won't break 100GB let alone 50GB in a month. Especially as the streaming companies have dropped the bit rate its not much of a thing unless it is your sole device for a family.
Also, here's the catch for later:
Vodafone’s move is not free for the company, he said, since “any increase in data usage will have some cost impact”. The plan would only pay off in future if people who have unlimited data realise its benefit and move to an unlimited data plan in future.
Which means you want unlimited, you're going to have limits on your speed to less than 10Mbps. Which is more than enough for the majority, its not exactly good for me as I want a faster upload speed as well so I'll not be taking the offer should it appear for my 4G broadband.
While I'm not against the gesture, managing bandwidth usage on a mobile network is not the same as managing it on a fixed network because the level of contention on the radio interface depends on the number of customers on a cell and what they are actually doing so if you've got a high rate of HD streaming in an area then other activities may slow so this may backfire on them.
Unless you are on an ancient contract with your fixed broadband provider you are unlikely to be subject to a cap (and if you are I'd call and get that changed as you could save some cash too)
On fixed networks it's easier to manage as you look at the peak utilisation on links in your network and ensure the capacity is in excess of this.