Great posts, but yours has one ridiculous statement:

Copyleft obviously is an imminent threat to proprietary software

To make non-ridiculous it needs to be continued as such:

that relies on copylefted software such that the proprietary software is arguably subject to the conditions of the license the copylefted software is distributed under.

Just in case anyone would read the post and become deluded into thinking copyleft threatens proprietary software in general, let alone imminently.