With all due respect, I don't think the question is "how do we simplify SPAs?"; I think the question is "why do we need to run any app in a browser?" Look at the work associated with the "Next Billion Users" project [1]. Most of our assumptions about how-and-why are merely based on luck-and-whim. We didn't arrive at SPAs through some elaborate Grand Design. I just don't understand what a web application offers that cannot be had with a straight client-server application. Are walled gardens and app stores really tyranny when the garden is limited to a certain company/domain? For example, if my bank offers a client-server app that is walled off from the rest of the Internet, should I consider this stifling? If I was working for this bank and developing their client-server app, should I consider this a step back in my career?
My perspective on this question is that its convenience. If the app is something relatively simple, I just want to use it on the browser and not have to go through the steps of installing software that I probably will only use once. Also using a webapp is less dangerous than installing a program in the sense that it can't read your files, install malware, etc. I can relatively safely use an unknown webapp with confidence, but I would only install a program on my actual computer if I trust the author of it.
[1] https://99percentinvisible.org/episode/the-next-billion-user...