You can split off business units like their hardware sales or dev studios, but that isn’t going to reduce their storefront market share at all.
Are you suggesting that they just split users up randomly? That would be probably worse for consumers- suddenly the friends and communities people have built up through Steam would be fractured, and users would look to find ways to get around it.
Split up by what publishers they have deals with? Well then those new companies would only be indirect competitors, not to mention that would also be worse for consumers as I’d have to suddenly make a new account with each new platform just to keep accessing my current library.
Like… How do you want to split them up in a way that doesn’t hurt consumers and publishers more than it helps?