[pessimizer]

Because it pushes work onto the client, and the client has an indeterminate number of applications competing for the same indeterminate speed and size of CPU and RAM. You know exactly what your situation is on the server, and you can decouple data and presentation without putting them on different sides of the wire.

[fixermark]

The counter-point is that (a) the number of applications competing for resources is semi-determinate, constrained by the user's patience and willingness to upgrade and (b) the number of users the server must support is similarly semi-determinant, constrained by the popularity of the service, size of data a user could want to manipulate, and the desired speed of those manipulations. There's an argument to be made for the economy of scale of pushing some of the presentation work to the side of the wire where there is a user who wants to consume the data; I wouldn't want to have to use, say, Google Spreadsheets as a server-side-only product.

Note that we can reduce this line of thinking to absurdity if we substitute JavaScript with, say, video; some clients are incapable of rendering video, some clients are constrained in the size of video they can render. Do we therefore never stream video down the wire, and require the client to grab the whole video element wholesale as a single request-response? Of course not; users don't want to wait that long. So developers make an educated guess on what their clients' user-agents can do and set a sane bar for size and scope of video content. A similar sane bar can be set for richness of client-side executable code interactions.

[pessimizer]

I'm not saying that you shouldn't architect that way, I'm just giving reasons why you might not.

>semi-determinate, constrained by the user's patience and willingness to upgrade

The user can vaguely determine it, but you can't. You can't guarantee a user experience without relying on the user not to be viewing your page on a two year old cellphone that they're simultaneously listening to music on and running three other js-heavy pages.

>the number of users the server must support is similarly semi-determinant, constrained by the popularity of the service, size of data a user could want to manipulate, and the desired speed of those manipulations.

You can assign whatever arbitrary amount of resources you want to your site on the server side, and allow in whatever limited (or unlimited) number of clients/visitors that you want to, so it's completely deterministic. The amount or resources utilized (whether on the client or on the server) will likely not be very deterministic, but you can have algorithmic problems whether on a thick client, on a backend data server, or on a backend presentation server.

[ksk]

Well said. Too many websites these days are JS heavy.