alt Hacker NewsI heard the same thing for Discord last month, and reddit and Twitter a few years back. It kind of worked for Twitter due to be outstandiningly bad, but it still didn't "kill" Twitter in the colloquial sense.
I don't see it going down any differently with Steam. It may take a dent and open up a competitor, but it won't do a move so catastrophic that it losses its leader status from that alone.
"pretty quickly" is a few years, not one month. Chrome dominated the browser market pretty quickly even though the richer bigger company microsoft already had most of the browser marketshare, and that was 3 years. Before those 3 years it seemed like nobody would be able to make a dent in microsofts monopoly, and then it was gone in just 3 years.
If steams fumbles as hard as microsoft did with internet explorer they too could be mostly gone in 3 years, replaced by a giant competitors product.