This is what I tell potential customers that are evaluating our product that creates single-page web applications where the JS dynamically creates all content: if you want search engines to crawl the content, then you're not using the right product. It's a surprisingly easy decision when viewed in those terms. Most SAAS products that likely would be implemented as SPAs do not care about public search engine access, and are only used by B2B customers where you can dictate a certain minimum standard in terms of browser versions (IE).
Everything else should be a traditional static web site with "decorative" JS, or "progressive enhancement".
Everything else should be a traditional static web site with "decorative" JS, or "progressive enhancement".