I meant a bit like: Let's say you have 2 mobile phone operators in your country ( duopoly ) we are ok that for example using SMS for banking interaction ( second factor etc )

I think this is a process; and somehow slowly people accepting those levels, and in a society it becomes normal ( to have whatsapp for friend group, to have facebook for family photos etc etc ) and you are being left out eventually if you are outside of those norms.

So it is not so different for bank to require something like google provided software.

I think if we accept that market concentration for essential services cannot alawys be avoided, there must be an obligation for these companies to provide those services to everyone.

The difficult part is how to guarantee this right without opening the floodgates for all sorts of scammers and organised criminals.

We need some sort of due process proportional in cost to the effects of account terminations (or rejections) on people's lives.

Isn't this how utilities are regulated?

In the UK some utilities do have a legal duty to supply.

I'm not familiar with the details though, so I have no idea what happens if someone is accused of having violated their terms of service. I think there are different rules for different utilities.