They take 30%, just like every digital storefront. You are also wrong about eliminating the physical copy mitigating the cost of selling there, as physical copies drive sales for these sorts of releases. They also allow small companies to increase their profit margin on each copy sold by selling a collector's edition for a higher retail price. If it was prudent for these companies to abandon physical copies they would have already done so, but it isn't. The lack of a physical copy of a niche Japanese release limits it's sales potential in the west, especially at full price. There are a multitude of examples of this. One recent one is Caligula Effect on Vita. Atlus tried to secure a physical release but was unable to due to licensing or contract issues on the Japanese side. Sales for that game were poor, the lack of a physical copy being a large reason why.
I feel like you made my point for me by pointing to Caligula Effect... A game that they elected to release to exactly one platform in the West.
Had they released on PS4 and PC and expanded their potential customer base, they would most assuredly have sold more copies.
Lots of developers have abandoned physical copies to great effect.
How many copies of PUBG: Battlegrounds have sold?
Hearthstone?
Prior to the PS4 version, Undertale?
Wait , you think if you eliminate the physical release all the potential physical buyers will suddenly buy it digitally? LOLOLOLOL!!!
If you want to play X game based on reviews, hype, word of mouth, whatever.... And it only releases digitally, are you going to buy it or skip it because you can't have your precious physical copy?
Digital sales may only make up 40% of total sales for a game... But if your game is digital only, then 100% of your sales would be digital.
If a publisher decided physical minimums for Xbox one were ridiculous and decided not to release the game on Xbox, how many potential digital sales did the publisher turn down?
By your metric, at least 40% of what he would have sold totally had he chosen to release on Xbox.
Again, you get none of the customers you don't attempt to get.