Thank you for this article. I believe that many WordPress business already consider themselves as startups or at least use tools and best practices of the startup world (like MVP). What I like in your article is that you set a focus on the fact that plugins and themes mechanisms were found to be a very good soil for startups. WordPress has democratized publishing and we may say it has helped doing the same thing for many people regarding startups. This is why WordPress startups can be side projects (in their form) even they have to be startups in their goals and practices (and you're right to highlight that).