> n a plain JSON world, I'd craft a single "user" endpoint, returning those three datapoints, and I would let the frontend handle it.

The main problem is that this is extremely, extremely expensive in practice. You end up in Big Webapp hell where you're returning 4mb of data to display a 10 byte string on the top right of the screen with the user's name. And then you need to do this for ALL objects.

What happens if a very simple page needs tiny bits of data from numerous objects? It's slow as all hell, and now your page takes 10 seconds to load on mobile. If you just rendered it server-side, all the data is in reach and you just... use what you need.

And that's not even taking into account the complexity. Everything becomes much more complex because the backend returns everything. You need X + 1, but you have to deal with X + 1,000.

And then simple optimization techniques just fall flat on their face, too. What if we want to do a batch update? Tsk tsk, that's not RESTful. No, instead send out 100 requests.

What about long running tasks? Maybe report generation? Tsk tsk, that's not RESTful. No, generate the report on the frontend using a bajillion different objects from god knows where across the backend. Does it even make sense with the state and constraints of the data? Probably not, that's done at a DB level and you're just throwing all that away to return JSON.

I mean, consider such a simple problem. I have a User object, the user has a key which identifies their orders, and each order has a key which identifies the products in that order. Backend-driven? Just throw that in HTML, boom, 100 lines of code.

RESTful design? First query for the User object. Then, extract their orders. Then, query for the order object. For each of those, query for their products. Now, reconstruct the relationship on the frontend, being careful to match the semantics of the data. If you don't, then your frontend is lying and you will try to persist something you can't, or display things in a way they aren't stored.

The backend went from one query to multiple endpoints, multiple queries, and 10x the amount of code. The frontend ballooned, too, and we're now essentially doing poor man's SQL in JS. But does the frontend team gets the bliss of not dealing with the backend? No, actually - because they need to check the database and backend code to make sure their semantics match the real application semantics.