[popo]

The lens would be optimized for canon EF mount, as this is the most popular mount and drives the most sales.  a 2mm thick coverglass would be used in the design of the lens.

 

As a minor variation, I'd think they'd design optically for Nikon F mount, since their mount is slightly further out than Canon. As such they could easily extend the lens body for others. Unless the optics can stick out much further than the lens mount for whatever reason.

 

Sure it can.  Aberrations with field dependence (coma, astigmatism, distortion, field curvature, and 5th order spherical) reach lower max values on a smaller sensor.  This would allow the pursuit of a greater telephoto ratio (=shorter) and the chief ray (touches corner of the sensor) defines the size of all elements after the peak in negative power of a lens.  For the new samyang, everything after the third element could be shrunk down.  A more tapered barrel could also reduce size.

I think the argument wasn't that it couldn't be made smaller/lighter, but it was to question how much smaller/lighter it could be. A 135mm f/2 lens would typically cover full frame sensor, and we would be looking at say 1.5x smaller image circle radius for APS-C sensors. Most of us aren't optical designers so don't have the experience estimate in detail, but the assumption would be the optics at the front would dominate regardless, and reducing elsewhere would lead to relatively minor gains in comparison.

 

A similar question often pops up on various forums, why aren't there longer focal length lenses (say, > 300mm) made specifically for APS-C sensors? The argument against it typically runs along the lines of the saving being insufficient to justify having two versions of the lens, so just make the one with the bigger image circle to cover both uses.