Telcos like VM and BT are super desperate to ensure customers maintain the telephone service.
It is allowing them to continue the same monthly fees ignoring the fact that the rental costs of analogue telephone lines and analogue exchange services no longer exist.
AIUI though, VM's 21CV service is still connected to the same old exchange equipment at the moment. If/when VM ever does provide a full VOIP service to customers, with full VOIP features, then it would be reasonable to expect prices to change accordingly.
In this 21CV half-way situation, which is neither full VOIP and nor POTS, I expect many customers will simply go mobile-only as the usefulness of having the landline as an independent means of backup communication has now been lost.