Has the Virtus Platform "come and gone"? A check of Sig's website shows all exchange kits out of stock. A friend asked me if I wanted to buy his 11.5" 5.56, and I don't know anything about this platform. Long time Sig owner, zero knowledge of the Virtus. Thanks in advance.

As far as I know the virtus is still very much alive.
With regards to the cal xchange kits - sig had an interview a few months back.
In it they stated that they're having a hard enough time keeping up with demand for the rifles themselves - so the kits are on the back burner for now.

Just like every other remotely desirable gun... Constantly sold out right now.