Old computers, before sandboxing and Windows defender and real-time protection, were more secure, because people were less likely to plug their bank account information, social security number, birth date, and home address into them.
At a certain point we have got to level with the idea that a smartphone is no longer a general purpose computer in your pocket. It's more like a cyber passport. It knows everything about you and authenticates formal activities.
So they weren’t actually more secure – they were less secure and less useful (setting aside the questionable historical accuracy of where popular online banking sits in the timeline relative to OS security measures in that claim). Maybe if we relax the made up constraint that a change must create 100% foolproof security, we can have a more nuanced discussion about ways to improve security.