I hate doing this, but I feel like I need to pick at each of your described hurdles, because I think each of them make assumptions or assertions that don't hold up.
1. You claim PC gamers do it for the hardware as much as the software. Let's assume the data backs that - it certainly seems like it's likely to be true. And I'm biased in wanting to believe it too, because I like to build and revel in the machines that run the games I own. What isn't true is that those same people, people like me, cannot also be attracted to things like Stadia.
2. Services like Stadia do not replace the many games that people play on the many devices that already exist. It's not a "one or the other" thing. They allow those devices to play more games.
The biggest flaw is in suggesting that casual gamers (a term which is flawed for many other reasons) wouldn't be a potential market for a thing like Stadi. Mobile game sales account for almost half of ALL game related sales. 48%, in fact. $76 billion in sales. A thing like Stadia means that people can play more games on their devices.
And let me say, games on Stadia play incredibly well on my iPad that's a few generations old. That's very attractive. Being able to play PC quality games on my iPad when I travel is worth every penny. I'd even argue it's easier to play games on Stadia than it is to play natively installed games. With Stadia, there's no downloading of the game, no installing, not time wasted waiting for updates. You just turn it on, and it works.
First, where you say "casual gamers", I think what you're trying to say is "people who play games on their mobile devices." You go on to describe the abilities that mobile devices have. While I won't dispute that, one thing I think you're missing is that services like Stadia make it even easier to play games on those devices that don't exist for those devices, or will at some future date, optimized to run on those mobile devices.
I'll probably beat this horse to death, but to compare: I was playing Cyberpunk 2077 on my iPad through Stadia minutes after it was available. It took nearly a day before I could run it on my PC, and after the first several patches I just stopped bothering. Granted, the game is a beautiful mess, but the point is: it was effortless on the iPad, and has been ever since. Not only that, but I can switch to my iPhone, or to my PC and pick up right where I left off. If I do it quick enough, the game just unpaused when I jump to the new device. And I can travel and still play. There's no way my PC, with its UV reactive liquid cooling is going to travel with me.
3. Stadia starter kit is optional. Stadia is free. Do you have a controller? Keyboard and mouse? A web browser? You're good. There is no required subscription fee. You buy the games, and they cost the same as console games. So yeah, if you have a device that can run modern browsers, you don't need to buy a console.
4. I assume when you mention latency, you mean "input latency" - meaning, the time it takes for the game to react to your button press or mouse movement. There are indeed hard limits to how low input latency can be. The game cannot update its entire model and render it in 0ms. It has to make calculations based on your inputs, then show you what changed. But that's not the only constraint. Consider the entire picture: a target on the screen moves, and you need to shoot it. If you're good, it'll take you about 100ms to react. Most people can't react in less than 150ms. It takes 5-10ms to transmit your reaction over USB. It takes the simulation any number of milliseconds to process and tell the monitor to redraw itself. Let's assume the processing time of the game engine is 0ms. The best monitors will add 2ms to the clock.
So, from your human reaction to the resulting frame, at best, it takes from 107ms to react to something on screen and see the results of your reaction.
And that's on your PC. No networking.
What does Stadia add? On a good connection, it'll add 20-30ms. To be fair, that's what I've seen on my pretty normal cable company internet connection over 5ghz Wifi. With most games, you'd never notice the extra time. Are you going to notice it as a pro gamer playing FPS competitively? Probably.
Your assertion that Stadia will die is about the most right thing you've said. Even with a market, Google tends to kill things seemingly at random. What will help it die quicker is if Nvidia's service is able to outperform Stadia in terms of simplicity and streaming speeds.
But saying streaming based gaming won't find a market reminds me a lot of what the cable companies and Blockbuster used to say about Netflix.