I know this is something you've been interested in for a while. Let me at least explain why I'm still hesitant.
Cost. A perfect implementation would work as seamlessly as FTP works today. To get there parts of SFTP-related code would have to be integrated, with licensing fees costing thousands for starters (see below).
Maintenance. If you've been following security advisories for browsers and operating systems, you know that new vulnerabilities are discovered seemingly every several months. These affect SFTP layers as well so these things have to be tested and maintained indefinitely because there's no such a thing as perfect security.
For these main reasons, built-in SFTP is not on the short-list I'm afraid. I'll reconsider if better options become available. SFTP could be offloaded to another application but then you might as well use a full-blown external client.