For me having a new stadium with over 100k attendance is a great idea. Then rebuild Old Trafford as the academy / Women's team stadium in a style sympathetic to the old circular bowl from the 70's and 80's but with better seating and accessibility options. You'd probably end up with a possible attendance there of up 25-30K which would also work as a secondary venue for other events.
The new stadium should have cheap standing, proper seating and commercial and executive options so that all different types of fan can attend. Putting the 2 stadia in close proximity should allow you to create a great destination for fans to watch a women's team match, then use hospitality before going to the "main event " of a home game.
it shouldn't need to be a case of prawn sandwich brigade OR "chipsngravy" from Macari's - in an ideal world both should be available if that's what you feel like. We owe it to older fans, those with disabilities to be much more inclusive, so redeveloping Old Trafford as the only stadium just doesn't work for me, unless we are willing to endure 3-4 years of disruption as we build one stand at a time.
The other factor is that as a brownfield site we are highly likely to experience significant ground work delays because of WW2 bombs or archaeological discoveries. If those delay a new stadia then it wouldn't impact the club as deeply because we go on playing at Old Trafford.
A new stadia would reduce parking while it was being built, but it could be 85% built with minimal disruption. with maybe the last "corner" closest to Old Trafford being built towards the end of a season so that there are no safety concerns.
4 or 5 years of reduced crowds will seriously hit our operating profits - so for me new stadia is the only way to go.