the device you're describing will cost like 2-3k, that'd be insane! I think what makes sense for Apple is to keep the dgpu option OR redesign the 15" so it can put that space vacated by the dgpu to good use. since the logic board is already designed and apple rarely changes logic board design language within a given design structure, it woudln't make any sense for apple to not source that option if it can make more money off of it.
How would it cost 2-3k for a display? A whole 27" iMac with a dgpu and a 3.2Ghz i5 is $1799.