The problem with one page websites is that you are giving away the benefits of having multiple pages which are all congregated in one word: diversification.
You can be more precise in the topic you will have to develop for each page. Also one of those benefits is the relationships you could create semantically and contextually between the pages. In a world that is being taking over by mobile devices, you will be delivering less scrollable content, improving user engagement and potentially better performance.
As a consequence of having multiple pages, you will be creating opportunities to develop vertical niches as your website grows. If you have a measurement plan, you will be improving the user experience by tracking and analysing the users' behaviour and making those changes accordantly.
In terms of SEO this is a challenge, it will be required to have a top to bottom strategy to create content that will be around a main topic. Then, the inner pages will be considered a supporting idea to the main topic. And so on.
I believe that you now can see that having subdomains is not a good idea if you want to develop niches later on. In the SEO world subdomains are considered separate websites. People have reported several times that their rankings improved once the took the decision to move from a subdomain to a inner folder.
Even though you have a multifaceted business, at the end of they day you serve a broader industry market, and that will be understood by search engines, especially Google.