What I see happening is a data only SIM being added, allowing for GPS and improving Continuity and Handoff between the watch and the phone. Not full independence from the phone but a few steps better than it is now.
Until Smart Phone market increase significantly, SIM in AW will not happen. This will required Apple to create another new product line, similar as iPhone vs iPod. There is just not enough people buying smart watch, yet, for Apple to invest 2 smart watch interior design products. There is good reason for Apple to use exactly same interior for Sports/Watch/Edition version of AW, because market is not there to create different interior sku yet. They will need to offer both SIM and non-SIM version, since a lot of people, like me, has no interest to allow carrier to rob me on another data plan. I don't need to pay for 2 data plans on devices on me all the time.
Non-AW smart phone company need SIM version to help steal Apple smart watch market share, not Apple, yet.
Even when that day is come, SIM smart watch, enough people could not live without smart watch, and for some reason does not want to carry their phone, Apple will likely ensure AW paring always required a iPhone, regardless SIM or non-SIM version. So they could continue ensure iPhone/AW revenue go hand in hand. Buying and using AW without owning iPhone will probably never come. This is how Apple protect their own product design and market.
How do one search app and install it in AW? One will need carry magnified eye glasses all time to read paragraph out of that tiny screen or scrolling like crazy.
Also it will not be data only plan if AW ever has SIM, other wise, phone call will be dropping like crazy for someone without iPhone, if allow. LTE phone call does not work everywhere and cut off like crazy when moving with tall buildings. If AW with SIM still required iPhone, why pay for another carrier plan when bluetooth/wifi with iPhone 95+% of time?