Seems like just a question of scope/mindset: they're talking about the
fixture as one component an installed
system while you're thinking about individual fixtures.
Something like the S4WRD makes sense because there might be at least a million
conventional S4 fixtures that could benefit from an
LED retrofit. With older arc movers, there's a much smaller quantity to start with, and an even smaller quantity where the rest of the (very complex)
fixture is in good enough condition to be worth dumping money into a retrofit. Add in the likely optical compromises, and there's just no
practical market for that type of product.