I think there´s a combination of factors behind Stadia´s troubled rollout and its complex reception among those of us who consider gaming to be one of our main hobbies:
1) The normal scepticism related to the widespread adoption of relatively new technologies. No one has tried to implement game streaming at this scale before. A significant percentage of gamers (even those who dwell forums like this) do not have the connection quality or stability required by such a service. It isn´t even available in my country, for example (Argentina). They haven´t been really careful with PR (negative latency, stacking overpromises, etc.).
I do believe this is the inevitable future some here from now.
2) It could be argued that Stadia began as an engineering project which was successful and eventually some service conditions and pricing were attached on it, and not the other way around. For those with optimal conditions, the tech is already there; but many crucial aspects seem to be rather raw (no exclusives, no studios, no achievements at launch, many technical caveats, etc.). They seem to be rushing it to beat xCloud in particular.
3) As ridiculed as the "Killed by Google" list is, IMHO Stadia could become a threat to established platforms. I welcome competition, but... what if Stadia becomes really mainstream? The YouTube integration and the no-specific hardware requirements could be gigantic.
Here, every day, we see an anthropologically interesting link between people and their platforms of choice (console wars, etc.). The scale and sheer power of Google, plus the potential innovation behind Stadia, means it could send shockwaves across this industry and our investments in the current platforms.
Even the gargantuan Microsoft (which, contrary to what the article says, is still bigger and richer than Alphabet/Google) has struggled to find a footing in the gaming industry during two decades. Sony has had a lot of triumphs and is widely beloved (particularly in places like this forum), but it is literally dwarfed by Google and heavily depends on the traditional model of selling hardware. "Traditional" PC gaming, as much as fundamentalists try to deny it, could be forever changed if streaming takes off. Nintendo is a relatively isolated case.
4) Google has dominated many markets. It has avoided antitrust manuals, emerging while the US government was busy attacking a much less powerful (but still seen as malignant) Microsoft. Search, browser, maps, mobile OS, online videos, calendar, pictures... it is becoming increasingly difficult to avoid using their products, many of which are de facto monopolies. They are almost unstoppable and even governments aren´t capable to firmly regulate them. I do not want Google in another aspect of my life. And I know that if this becomes viable, they could start a purchasing spree of studios and exclusives that could alter the landscape of the industry.