I think it is just a problem between theory and practice. In theory it is possible, but in practice it has proven to be so hard, that I am not sure if there ever was or will be a secure computer that is connected to the internet.

Another problem is that the complexity in modern systems is increasing and designing secure systems on top of complexity is next to impossible. I think Unikernels have a good chance of delivering secure systems from a software perspective, but it seems even modern hardware has enough vulnerabilities to thwart those plans.