As for DOS attacks, they are often bandwidth based, but they do not have to be and the best ones are not, they attack bugs in TCP/IP stacks, like the good old Ping of Death.
Not just bandwidth as I noted it has a lot to do with the amount of connections attempted as well, similar to the old ping of death.
It's enough to saturate a user (household), branch, or node with so many connection requests that it cannot all be processed in time. You can see this effect at the local level just by setting the number of connections used to a slightly unreasonable level in a torrent application. And that happens relatively frequently. If too many people do that on a neighborhood network trunk, it will then affect everyone on that network trunk. And some torrent clients attempt to mask their connection as other data types in order to get around ISP QOS shaping.
However, it need not be as all encompassing as a DOS attack (whether bandwidth based, connection based, or bug based) in order to affect gaming or streaming or any other service. Orders of magnitude less, in fact. And it doesn't have to be something that is maliciously or consciously done. This isn't something that needs to completely deny service, but just to introduce enough of a load that the infrastructure at certain junctions starts to take a few hundred more milliseconds in order to process it.
Twitch's service is entirely built on relatively quick live streaming (unlike Netflix) and here you can somewhat regularly see its streaming service have bubbles where stream quality is degraded and/or interrupted. This situation gets much worse if you enable low latency mode.
And that happens to people regardless of the quality of their ISP albeit people with good ISPs have this happen less often. But even still it can get to the point where it affects even people with guaranteed service contracts (expensive business internet lines).
Regular streaming has everything processed and delivered at an average latency of around 10-15 seconds, IIRC. Low latency reduces that but the latency is still in seconds.
And that's with a live streaming service that is much larger than Stadia (number of users) is currently but with demands much smaller than Stadia (significantly less bandwidth per user). Netflix isn't really applicable here as it isn't doing live streaming.
In order to achieve what you envision, the entire Internet would need to be reworked with infrastructure redone on every node of every backbone provider and every node of every ISP. Network trunks that service neighborhoods would also need to be entirely reworked.
I don't think Google has enough money or power to make something like that a reality.
Overprovisioning is a business decision that is never going away. Let's say your average user of a 30-70 Mbps connection is expected to average roughly 5-10 Mbps per day on average (it's quite a bit less than that for most neighborhoods) and maybe 15-20 Mbps during busy hours (like when people just get off from work, and likely less for most neighborhoods).
Now what happens if users with Stadia are suddenly averaging quite significantly more than that due to Stadia's bandwidth and latency demands?
What happens is that the average bandwidth per user calculations need to be redone and ISPs would need to adjust pricing accordingly (more bandwidth allocated per user) or implement bandwidth caps (same bandwidth per user, but now enforced with a cap).
So the reality for people wanting a good Game Streaming experience (at least WRT Stadia bandwidth demands) could either see their monthly ISP costs triple or quadruple (possibly more) and/or ones without a bandwidth cap will suddenly find they have a bandwidth cap which means their monthly ISP costs could balloon even further if they are charged for going over that cap (like Comcast, ARGH).
Although in Comcasts defense, the quality of service (average latency in games and connection service requests fulfilled time) on my internet has improved now that people on my network trunk are disincentivized from using a lot of bandwidth per month. Of course, that also means I can no longer stream nearly as much 4k video as I would like. Heck I can't even stream 1080p/60 video as much as I would like. So it's a mixed blessing.
And this is with me ONLY using Comcast for video streaming. I have another low cost (10 USD per month) and low bandwidth internet service for everything else which has unlimited bandwidth but is limited to only 15-25 Mbps depending on the time of day. And at certain really busy times it can drop as low as 6 Mbps. Basically can't do 4k video on this connection reliably and even 1080p/60 can't be reliably done. At the time of this posting, I'm only averaging 10 Mbps on this connection, yay.
I can't afford to do all the other things (game downloads, OS downloads, etc.) in addition to video streaming on my 1 TB per month Comcast internet.
Basically on Comcast I would have to pay a minimum of an extra 50 USD per month to remove the bandwidth cap if I wanted to use Stadia. If Stadia actually succeeds and a lot of people start using it? I expect that 50 USD per month cost to balloon to at least 150-200 USD for unlimited bandwidth per month.
So yeah, Stadia most certainly does not represent good value for money in my area. Many areas where there is no bandwidth cap, people often suffer from highly variable ping times (latency) for games depending on the time of day. This was extremely noticeable when I still did hardcore raiding in MMORPGs where variable ping times meant complete raid wipes and wasted time. Especially in FFXIV where timing for the Savage raids was very precise and timing windows were very short.
Regards,
SB