I'm struggling to have sympathy for people who sign up to an £x per month contract for 24 months which includes a 'free' phone and don't think to check after those 24 months are up whether they're still on a good deal.
Mortgages, broadband, television, utility bills and just about every type of recurring payment has some provision for what happens when you complete your fixed term and not a single one of them puts the price down automatically at the end of it despite you having paid off the cost of your Sky box or router. The costs of being moved onto your mortgage provider's SVR after your fixed term end will dwarf any mobile bill.
Not all mobile contracts are necessarilly bad deals. My other half recently got a Galaxy S8 on a 24 month contract at a total cost of ownership of only about £100 more than buying the phone alone. It's not exactly a chore to stick a reminder in the calendar to change contracts after 2 years.