On the path they've started with pipelining calls parametric on previous calls, they'll quickly realize that eventually they'll need basic transforms on the output before they pass it as input i.e. necessitating support for a standard runtime with a standard library of features.
I.e. imagine you need this:
promise1 = foo.call1();
promise2 = foo.call2();
promise3 = foo.call2(promise1 + promise2);
Can't implement that "+" there unless...
promise1 = foo.call1();
promise2 = foo.call2();
promise3 = foo.add(promise1, promise2)
promise4 = foo.call2(promise3);
You can also make some kind of a RpcNumber object so you can use their Proxy function to do promise1.add(promise2) but ultimately you don't want to write such classes on the spot every time. Or functions on the server for it.
The problem is even that won't give you conditions (loops, branches) that run on the server, the server execution is blocked by the client.
Once you realize THAT, you realize it's most optimal if both sides exchange command buffers in general, including batch instructions to remote and local calls and standardized expression syntax and library.
What they did with array.map() is cute but it's not obvious what you can and what you can't do with this, and most developers will end up tripping up every time they use it, both trying to overuse this feature and underusing it, unaware of what it maps, how, when and where.
For example this record replay can't do any (again...) arithmetic, logic, branching and so on. It can record calling method on the Proxy and replaying this on the other side, in simple containers, like an object literal.
This is where GraphQL is better because it's an explicit buffer send and an explicit buffer return. The number of roundtrips and what maps how is not hidden.
GraphQL has its own mess of poorly considered features, but I don't think Cap'n Web survives prolonged contact with reality because of how implicit and magical everything is.
When you make an abstraction like this, it needs to work ALL THE TIME, so you don't have to think about it. If it only works in demo examples written by developers who know exactly when the abstraction breaks, real devs won't touch it.