It's interesting that this has been brought up again. A couple of years back I interviewed Gary Hetherington for a piece that was due to go in a book about the history of English schools rugby league (remember Sheffield Schools U11 played at Wembley in 1993). The conversation naturally wandered onto Eagles matters and Hetherington reiterated his opinion that the only way a Super League club would work in our neck of the woods would be a merged South Yorkshire "franchise".
Of course he would say this as he thought it was a good idea in 1995, but his argument is pretty much that Sheffield alone is just not interested enough in Rugby League. He did admit that neither club's fans would buy into it, and therefore it's a non-starter.
It may have worked if neither the Eagles or Donny existed, or if Donny hadn't existed when the Eagles started. Now… unlikely.
Eagles do have a problem in terms of generating and sustaining crowds, but in the old division one and SL days our crowds hovered around 4,000 - not amazing, but comparable to people like Salford. At the time of the merger Huddersfield were getting no better crowds than the Eagles. They've only improved in recent years 'cos they've worked stupidly hard marketing themselves.
To be honest I'd rather see the Eagles play games "on the road" in the Midlands than play any in Doncaster. No offence to Donny fans, of course