As @
Qwerty133 said, great if it's a nationalised railway, but it isn't.
Yes, Stagecoach run both franchises, so great for cross boundary thinking and all that, but do they share profits too? No. I didn't think so. Are they using East Midlands profits to prop up Eact Coast? No, I didn't think so.
I can see the merit in the idea, but I don't see any other industry where this happens as much. I'm sure I've seen a couple of airlines do it in the past but bus companies don't, cruise lines don't either. Hell, even local councils don't share bin lorries or whatever.
Yes perhaps they should, but each company are "each to their own" - maybe shooting themselves in the foot, yes, but why should passengers using East Midlands have one coach short to help out a bunch of strangers using another railway line? I rarely see East Coast trains helping out East Midlands...
We don't live in the world where we share like this any more.