Other people are making good counter arguments, so I'm just going to address one bit:

You can also look at how despite charging a 12% platform fee, Epic Games Store does not sell games 18% cheaper.

Epic hasn't been running their game store for very long, and they've been operating it at a loss to secure market share. They lose hundreds of millions of dollars a year on their store. This is mostly due to them buying exclusive rights to games, but my point is that the EGS is not a successful, self sustaining business. Epic taking a 12% cut doesn't mean that 12% is enough money, because their whole business model is about losing money to attract users.

You also have to remember that the storefront cut is an upfront cost with an unclear long-term cost. Valve is promising to always host the game and cover the bandwidth for every future download and update, no matter how many updates or how many times someone downloads it. Not to mention that they also will host mods, provide matchmaking, video streaming, and many other benefits.