Not that big of a problem
As per title, I don't think this is really a big problem.
If you're making that many calls outside your bundle that the increased call charges affect you significantly, then you're almost certainly not on the best value bundle for your level of usage and should switch tariff to one that includes the volume of calls you make.
In terms of voice calls the only people this will affect are people who go very slightly over their bundled calls each month, and if they only go over very slightly the cost increase will be minimal.
As for the other stuff, like picture messaging, who actually sends MMS messages these days anyway?
I've stuck with T-Mobile/EE for a number of years now for only one reason really, the fact that their data usage is capped. You get what's included in your tariff and then you're cut off, unless you actively choose to pay to add more on. Admittedly their data add-ons are horrendously over priced, but at least I get to choose whether I pay it.
I find this far more preferably to many other networks, where they charge either a fairly hefty per MB rate (1p per MB sounds cheap, until you realise it's £10 per GB) or a daily rate (£1 a day sounds good unless you accidentally use all your data within the first week of your billing cycle) for any usage over your bundled data, which could potentially lead to some fairly inflated bills.
Of course most smartphones now have very easy methods to switch off or cap mobile data built right into the OS, but back in the days of Windows Mobile 5/6 devices and the like such an option was either third party or did not exist.
It only took one massively inflated bill from O2, after my Windows Mobile device had lost it's WiFi connection and had eaten through a GB or two of data, for me to realise having the network automatically cut you off rather than just keep billing you was the way forward.