The model itself isn't really that bad (though it would work better if they offered a flat rate subscription that just unlocks everything the way Viz does with Shonen Jump). The main problem is that they currently lack the secondary infrastructure that is needed for this model. When this type of model is used in Japan, if you like something enough to plunk down cash for it, you can go over to Bookwalker or Amazon or something and just buy the volume once it's available.
The problem here is there's no indication that AlphaPolis plans or has any intention of collating the chapters released in the AlphaManga app into volumes for sale digitally or otherwise in English which means outside of Isekai Tensei Soudouki there's currently no way to make a purchase to own (you could buy Re:Monster from SevenSeas, but it's going to be a different TL/Lettering than the version in the AlphaManga app). Given that a number of titles in the app now have enough chapters released that we could easily be on vol 3 or 4 of the mangas, it's honestly not looking good.
If they at least announced their intent to release digital volumes for sale on storefronts, I could at least get behind the model as it stands now.