Just like desktop computers they will reach a point where, for most people, they're 'good enough', even if they're really slow.
For some people, particularly the elderly, the young, and the poor, they'll use it until it literally stops working at all because they literally cannot afford otherwise.
Cell phones are a literal lifeline device, to the point that emergency calls (at least in the US, and probably in any civilized part of the planet) //WILL// always work; without unlocking the phone, without the phone being on a payment plan, as long as the device has the power to transmit that distress need.
It is not difficult to imagine how issues with the security portion of the layer could allow anything from a computer virus to DDoS local infrastructure (jamming), to targeting specific individuals (lock out their ability to call for help, bug and track movement), to picking out individuals that are alone and in vulnerable moments. Those are attacks I thought of just typing this comment and are not an exhaustive list.
This is why the security updates either must be performed by the companies in question through the actual use lifetime of the device: not just by it's first owner, but all the way through to it's last. Or failing that, they should be required to both release the sources and provide everything necessary in a legal sense for other entities in the community to perform the security updates they are unwilling to do. Carriers should also be liable for failing to (offer to) distribute said security updates to devices on their network (it is in the interest of everyone that they do this and do not charge for it).