While I agree that it shouldn’t be allowed to enforce price matching, I don’t believe for a one second that 99% of publishers would pass what they’re saving onto the customer. A big AAA title priced at £59.99 would still cost £59.99 even if Valve suddenly decided taking just 15% was enough.
Valve does have a very dominant market position, but at least the PC (whether it runs Windows or Linux) is a very open platform that doesn’t discriminate between storefronts. So a publisher can always decide to not agree to Steam’s terms and only release their games via another store or even release it standalone. I agree that this would likely lose them sales over releasing their game on Steam, but if it’s marketed well and the storefront they chose doesn’t suck, customers will still come to them.
The main reason Steam is as popular as it is, is because it’s the best from a customer experience standpoint, not because they have an enforced monopoly like the storefronts on console platforms. The Epic Launcher, Uplay and Origin (or whatever they’re all called now after renaming on what feels like a yearly basis) lack a lot of features in comparison (depending on the launcher that includes properly working cloud saves, hassle-free Linux compatibility, easy mod integration (Steam Workshop), sharing your screen to play local co-op games online, just to name a few).