Just build a normal website and let my browser do what it was built to do. The APIs that allow SPAs are great and useful. The Pixlr online photo editor has a good reason to be a SPA, and that works great for it. Your forum/email interface/management API would work better as a normal website.
Good for you with your fast, always-on internet, I guess. Some consumers have to deal with expensive and unreliable 3G internet (that downgrades to 2G occasionally) - an offline-capable email (or management) SPA is indispensable for them. Sure, you could write 4 native offline apps for MacOS, Windows, iOS and Android but that is a bigger investment than targeting browser standards.