That is the tool for basically doing anything productive as a business... it isn't as if they've done some about face at the emergence of genuine competition - they've always been unusually well liked (especially considering the industry).
And fortunately for Valve even 3 years after release Epic Games Store don't even have half of Steam features. Also Epic is so consumer-hostile (or as they call it pro-developer) that they won't ever implement player-reviews into their system.
So it's not that hard for Valve for keep their mindshare. Far larger danger for them is Microsoft with it's Xbox game subscription model and unlimited amount of money.
Meanwhile the large developers, the ones who are making $50M+, those developers have enough pull to make the user install a custom client like Origin. That’s why Valve cuts them a better deal.
Disclosure: I work at small indie PC gamedev studio.
I just point out how Valve changed rules for big companies back in 2018. So they have different revenue share compared to both Apple / Google and consoles.
However, there are many times more small indie titles than there are big successes, and it's the big titles that draw users to your platform and bring in the money.
If Valve flipped their model, those indie titles would be even more of a net drain on their profitability, while there would be more incentive for the big titles to release on their own platforms.
Its tool for doing that is consumer sentiment.
This all plays into Valve's business strategy. They're fighting to justify ye ole 30% cut.