I agree with some of what you said, but having produced websites using Shopify, various standalone PHP shopping carts, and Wordpress plugin shopping carts myself, I think there are a few things I want to clarify:
First, of course Shopify websites can be on their own custom domain. One need only look at the example websites or read the docs to know this. How good a job Shopify does with regards to SEO analytics vs. Wordpress with your preferred SEO plugin may be open for debate, but Shopify does have all the basic SEO bases covered.
I do agree that 3rd party hosted services make business owners reliant on the service provider for core updates / bug fixes. However, this is also true with 3rd party off-the-shelf plugins. The difference is that the business owner can choose to have a developer fix issues with the software themselves or opt to upgrade / downgrade at a time of their choosing. This difference is true only for core shopping cart functions as the UI can be updated at will with Shopify and other similar hosted services. And you better believe that these services will fix a core shopping cart bug ASAP as it is their bread and butter. Hosted monthly services must keep providing consistent and timely support or customers will leave. If you make a one-time purchase of an off-the-shelf plugin, the developers don't necessarily have a strong incentive to support the software.
The developer pool for any given platform is indeed important to consider, and I agree that you can't go wrong there with Wordpress. Most of my work involves WP and I highly recommend it to most clients. However, I also believe in using the right tool for the job and that each case is different. If a client came to me and said that they want a content-driven website and a shopping cart as a side feature for a few products, I would probably recommend WP. If, however, the site's core function is ecommerce, and especially if the features they would like are more advanced, I may not recommend WP in their case. If someone just wants a quick ecommerce website to test out their idea with minimal development effort, services like Shopify are the way to go. The upfront costs are minimal, especially if they use an existing theme. Having a UI developer make a custom theme would be cheaper than setting up a WP site with an ecommerce plugin and custom theme.
So again, it really just depends on what the details of what the OP wants to do, and I agree that future plans should factor into it. But proving the product in the market need not involved a lot of custom development up front, and I am hesitant to offer a catch-all solution when I don't even know the problem.