It's not just the last mile either – it's physical distance that's deadly to this approach.
My ping to a server inside my ISP in London (and I'm in Oxford) is 18ms on average, on a fast connection. I play games regularly at home where my ping is around 20ms, which lines up. I'm in Oxford, which is really damn close to London. The further away from the datacentre you get, the worse your ping will be. It's not even just about the speed of light in fibre optic cables (31% slower than in a vacuum) since our country is only small, but each additional piece of networking equipment on the way adds more latency, plus every time you press a button using Stadia, your latency is effectively doubled.
Normally you move, your computer renders that immediately and sends the movement to the server. With Stadia, you press a button, which is sent to the remote server (18ms minimum for me), which renders the change and sends it back to you (another 18ms minimum). Now, 40ms doesn't sound like a lot, but that's best-case scenario based on them having a server inside my ISP. Which they won't.
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