IMO, money still not the biggest issue. The main point of having the Apple watch is to extend the iPhone's function (e.g. able to get notification / msg / call without pulling out the iphone). And most likely the user want to extend the function of the normal used phone, but not buy an extra phone to use the watch (which means doing nothing to the normal used iPhone).
If we put the normal sim card in the old 5s, than why we still carry the 6+ around? The 6+ basically doing nothing, but just serve as an backup device. Most likely the user want to use the 6+ as the basic phone, but then the watch can't pair up with it, and can't extend it's function, then what's the point of having the watch?
So, I don't think that carrying 2 iPhones is a good way to solve this problem. Unless the user only want the watch to serve as a watch, but not really a useful smart watch
.
Or may be use the phones in a more complicated way. e.g. Forward all call from 6+ to 5s, it may able to solve part of them problem.