Last time I removed malware from my grandma's smartphone, I made sure to install Signal to it. Sometimes you just got to demonstrate to them how low impact a change is. Many elderly are frightened by the idea that their experience changes, on a device they depend on but hardly understand. So demonstrate, rather than trying to convince them of something they fail to understand. The worst that can happen is that they tell you to remove it again, but in case of my grandma, she comfortably uses Signal. Just make sure you send some example messages back and forth, so they understand how similar the workflow is.