I would like to see Downton during WWII. Of course, the cast would need to be in old age make up or just gone and replaced with a new staff and younger Crowley's.
On one of the DVD's of Downton Abbey there was a behind the scenes documentary where they spoke about how so many estates like this were being sold after WWII and different families were making these estates their homes. If they wanted to be realistic they could go this route, but I think this is what Matthew is trying to avoid.
I think he thinks if they cut their spending and try and live to be more practical, Downton may survive for future generations. I think that would be a smaller household staff. I get the sense Robert is having a difficult time with all this. Not having a say on how his granddaughter is to be raised and now not able to live the extravagant lifestyle he is use to may be too much for him to deal with. I think Matthew having a stake in the future of Downton is a good thing. It could however become a wedge issue between him and Mary though.
'There are three sides to every story. My side, your side, and the truth. And no one is lying. Memories shared serve each one differently'
-Robert Evans-