Two hundred meter trains sounds nice for Sheffield - Manchester (certainly a much easier way of dealing with busy trains than spending hundreds of millions of pounds on a route through Bakewell to take a handful of East Midlands - Lancashire people off the current route through Sheffield!)
But the six coach 185s are already fouling Meadowhall and half the stations east of Doncaster, so I don't know how much longer you'd want trains before a decision is taken to abandon the through service to Cleethorpes (which certainly doesn't need eight coaches, unless it's a warm/dry Bank Holiday Monday)
There's also the problem that, whilst an eight coach train would be great for city-to-city journeys, and provides plenty more seats, it limits the scope for those services to stop at the "local" stations, which means the much shorter local trains have to pick up more of the strain
I suppose that my answer really depends on whether we are ever going to get more than two semi-fast services per hour from Sheffield to Manchester - if we are limited to that number then, sure, let's go for trains that are as long as possible, even if it means losing the through trains for Doncaster etc, but it's a trade off