I disagree. The structure and/or implementation of email is not "causing people to use it sub-optimally".
If you want to manage projects and tasks, there are very good tools available for accomplishing that goal. But don't expect all your clients to start using your management tool because you think it would be "optimal". Email is convenient and easy for them, and they don't have to learn a new system to use it.
If you want to send files to other people, there are already specialized tools for that. Tools that have no size limits, that don't limit file extensions, etc. But don't expect all your clients to start using those tools unless they have a good reason to. Email is convenient and easy for them, and they don't have to learn a new system to send files with it.
There are endless other workflows for email, most of which have a specialized alternative, an "improved" version of the underlying technology. When an individual gets tired of the inadequacies of a certain facet of email, they will find it and start using the alternative. Until that point, they will use email because it is "good enough".
People actually are sub-optimal. I meant to start riding my bike to my new job, the money and health benefits would be optimal. But I'm not optimal, so I'm still taking the train. Until I find a compelling reason to start riding again, I'm going to keep taking the train because it's "good enough". That doesn't mean that the train system needs to be fixed.
Frankly, I think the whole "let's fix email" debate is a complete non-starter, and the bevy of related blog posts popping up for the past two weeks is a whole lot of noise from people who haven't actually thought the complexities of "the problem" through, at all.