The story I heard was that containers let you use less memory and better share the kernel and CPU compared to Virtual Machines, such that you could run more applications on the same servers. This translates into direct cost savings, which is why large companies with large server farms were willing to pay their engineers to develop the technology and transition to the technology.
In terms of security, I think even more secure than SEL4 or containers or VMs would be having a separate physical server for each application and not sharing CPUs or memory at all. Then you have a security boundary between applications that is based in physics.
Of course, that is too expensive for most business use cases, which is why people do not use it. I think using SEL4 will run into the same problem - you will get worse utilization out of the server compared to containers, so it is more expensive for business use cases and not attractive. If we want something to replace containers that thing would have to be both cheaper and more secure. And I'm not sure what that would be
In terms of security, I think even more secure than SEL4 or containers or VMs would be having a separate physical server for each application and not sharing CPUs or memory at all. Then you have a security boundary between applications that is based in physics.
Of course, that is too expensive for most business use cases, which is why people do not use it. I think using SEL4 will run into the same problem - you will get worse utilization out of the server compared to containers, so it is more expensive for business use cases and not attractive. If we want something to replace containers that thing would have to be both cheaper and more secure. And I'm not sure what that would be