You are correct, MCID is a random, unique-per-app identifier. It will not match across applications, even if those applications share the same Experience Cloud Org ID. Additionally, the MID will also change if the user uninstalls the app and reinstalls.
You can use something like customer attributes or customer IDs to "group" devices. However, this isn't going to deduplicate devices in your reporting, It will simplify allow you to deliver the same experience to that user across the apps or devices.
I don't have access to roadmap, but I'm sure we'll have some kind of soultion at a later date. If you look at the Device Co-Op I believe this is the first step. We are trying to stop identifying browsers and devices and identify individual people instead.