The problem with this idea is that the user's browser's compute is not "free". Offloading the computing means the users have a worse experience, which will affect your userbase and page rankings.
Like most things, there is no simple right answer, and it depends on what you are doing. But blindly assuming the experience will be worse using CSR is as silly as assuming SSR will always be worse as well.
Does it though? Loading a webpage barely registers in cpu usage etc on a reasonably modern device
We have 20+ core consumer-grade CPUs and double- to triple-digit RAM and the internet at-large (read: Web2.0) still runs like it's the 1970s.
CPU usage is not the problem. In most cases the problem is latency including the network latency to issue and return a substantial number of remote API requests across the Internet to get the data necessary to render the page.
In many cases unless you make page specific APIs that aggregate all the necessary data into a composite object on the server side this is the number one thing that slows things down. Network turnarounds are expensive but they are a lot less expensive when made inside of a datacenter than from a thousand or more miles away.