Sure, Google might get a millisecond or two less in their apps, but who in their right mind has the engineering time to implement this very, very small optimization when there's many other things to be done that probably have a higher priority?
Take a site like reddit, where the HTTP request can take a second or 2 to return when there is a really high server load.
Reddit could return HTTP 103 hints to the clients to start downloading useful assets ahead of time so when the HTML payload finally arrives everything is ready to start execution. And if this does become a standard, it can get built into web servers and will be as simple as adding a few lines to a config file.
Obviously the "correct" solution is to reduce the time it takes to return the HTML from the server, but if it can improve load times for a nonzero amount of people for not much work, it's a win in my book (even if it's not ideal).
Now, implementation of this by browsers is going to take a long time, and then we will see incompatibilities trickle down all the way, with Safari taking a few years after everybody else has it and IE/Edge also taking forever.... I should rather work in a farm and harvest tomatoes...
Most modern SPAs also have a static index.html which shouldn't take long to serve anyway.
This is going to be abused by cargo cult brogrammers who heard that "103 will make your site faster!!!1" and put every single resource as a 103 making the time to 200 even longer than the time you actually potentily gain.