I think the real problem that Apple and Google have to overcome for their watches to succeed is convincing people that they don't replace wristwatches, instead they are smartphone accessories. People have long given up wearing wristwatches and aren't looking to simply replace them with something else. The Apple and Google watches are really smartphone accessories, and the question to be answered is, do people want or need a smartphone accessory?
My belief is that watches need to be more than accessories for smartphones, and to achieve that they need to communicate with as many other devices and services as possible. The watches need to be accessories for desktop and laptop computers, appliances, cars, web browsers and more.
Note that right now Apple Watch needs an iPhone to work and Android Wear needs an Android phone to work. I don't think that watches need to be complete stand-alone devices, but I do think they need to communicate with more than just smartphones to be of the best use for enough people. Put another way, right now the potential market size for these watches is at most the same size as the respective smartphone markets. Do you want to be selling devices that have a market cap right from the beginning?