If Valve started to routinely do Bad Things on Steam they'd be gone pretty quickly. Many would go to GoG, some just stop buying games. Bad Things do occasionally happen (bad things like those "oops, we don't actually have licenses for the music used in the game you bought" revokes), but Valve keeps succeeding in keeping it to a rather low background noise level. Competitors have two decades of being that good or better to catch up. You can't buy trust, you can just put money into not losing any of the trust that grew over time. When competitors have done that for two decades, Valve, unless they fail in the meantime, will have even more.
I 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.
Which of them are privately owned? In a publicly traded company, there's an inherent logic that people who believe that the company can get by with squeezing customers a little harder will end up with higher projections than those who think it's better to get by with a moderate approach. Price goes up with the bids from the squeezers and occasionally a moderate will sell until eventually the squeezers own an identity-defining fraction. Valve only is what it is because of the ownership structure, its closeness to being bootstrapped (I assume that in reality ownership is a little more complicated, but close enough)
We could also call those squeezers "optimists", and publicly traded implies ownership by the most optimistic (well, the most optimistic who have money to invest). Leading to behavior patterns that could be described as suicidally chasing the most unrealistic money making projections. (and founder majority stakes are surprisingly susceptible to falling in line with those optimists, because those owners still don't want to see their valuation going down, doubly so of they ever started borrowing against their stakes)
"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.
The Reddit blackout is coming on 3 years old now. The twitter kerfuffle is almost 4? I'm not holding my breath.
And yes, chrome is a great example. That came right on the legs of Microsoft losing an anti trust case. For something that seems so quaint in 2026. I miss when regulations had teeth.
It being a good service is secondary.