Their policy is not that you aren't allowed to sell your game cheaper on another platform, their policy is that you can't sell Steam keys on other platforms cheaper than you are selling the game on Steam. Basically, you can't use Steam's infrastructure when undercutting "Steam customers". Games that are on Steam go on sale on other platforms when they are not on sale on Steam all the time currently.

Then they would just simply stop giving out free steam keys for off platform purchases. Depends on how many people buy from publisher site because they get to keep their games in a single library, it might end up with the game publishers getting less revenue overall.

Sure let them sell cheaper on gog or epic. But what about the Free steam Keys they get? Those should still be the same price or will steam change it to non Free Keys and instead Charge the 30% they take so that These Keys can be sold at any price any where? Tbh even If the game is 60$ on steam and only 45$ on epic.. I should still buy on steam.. And i suppose most others aswell