Looking at your age, kudo's for the article, but I think there is some misleading info in there:
When I say that we don't need a backend, I mean that a lot of applications that we are building right now (SaaS, AI Tools, etc.) don't really need a separate backend.
Separate to what? The frontend? You definitely need something other than the front-end for most applications. Your point seems to be that you don't need a custom built backend, not that you don't need one at all.
So first, How do you really build an application without a backend with Next.js 14? Simply, you use a lot of third-party APIs to do so ...
Wait, you don't need a backend, you only need third-party API's. How is an API not a backend?
Other than that, if you are using disconnected generic solutions you really, really have to pay attention to authorization and sanitization. Great that you can authenticate using a service, how is that going to handle authorization in another? Also, the frontend is not to be trusted, so any solution that puts authentication or sanitization in the frontend is highly flawed.