Are sd card slots cheaper than 128 GB of flash storage chips? I’m not sure, but yes, probably. You would also need to factor in the additional complexity of allowing physical access to the slot, which would take some additional designing and a few more components. The sd card itself will probably be a more expensive and slower than integrated flash storage. By contrast, it is probably extremely easy to just shove some flash storage chips in a phone. Still, I agree that it sounds like a worthwhile tradeoff for me.