Very valid observations but may I offer another reason why I reckon a massive stadium would be a part empty embarrassment.
I suspect attendances peaked when utd were in their pomp and went to 78k they looked well capable of filling a 100k stadium but interestingly never built one - they must have known something. In a similar fashion the other global entity, the RS, have gently nibbled away at anfield rather than go the swanky huge stadium route despite a site, a design and planning permission. Finally, all conquering Man City always with a strong core of loyal fans like us are seemingly incapable of filling the bigger Etihad despite monumental success we can only dream of. I just think a realistic uplift in new match goers tarkes us to the mid 50k region
If we could build a variable capacity stadium that always looked full despite the attendance I'd be up for a huge capacity it but I don't know if such a thing exists in addition every club building a new stadium has turned to absolute cack so I don't see BMD as the panacea many seem to believe