I've been running a low-ish volume site since 2014. It gets a YoY increase in traffic of like 20%. We didn't really do much SEO, at least not consistently enough that it warrants a consistent 20% YoY increase.
Then last year I made a English version of that website. Traffic has been very very slow initially, but slowly and steadily increasing. I suspect if we don't do anything to it for a couple years traffic will just keep increasing just like the original.
I generally have better things to do than to jump onto the cargo cult that is SEO, but the GP's description of how Google throttles web sites is exactly what I observe anecdotally, and is exactly how I hypothesized what Google is actually doing (without doing any SEO research).
Of course, there's not going to be any conclusive "proof" of such claims unless they admit it themselves, or unless you start an antitrust lawsuit against Google.
I definitely agree it's not a good idea to route all traffic to a new site that hasn't "proven" itself through some test of time, but that's more evidence that they're probably doing this throttling.
Do you really think it is feasible/a good idea that the moment someone makes a new site google should send all the traffic to that site?
Please THINK how easily that would be abused.