Have OAOs benefited passengers? Unquestionably yes, for those that use them.
But that wasn’t the question. The question was “Do open access operators benefit or hinder our railways?”
The answer to that, in my opinion, is that on balance hinder, but it is not necessarily their fault.
Why?
1) they are not on a level playing field with their rail competitors. They do not pay the full track access costs that franchised operators do. Ultimately, this causes net revenue loss to the industry.
2) by their nature, they tend to cause the timetable, and thus the ‘system’ for moving passengers, to be less efficient than it could be. Short trains is one example, taking up marginal spare capacity is another (which causes reliability problems for everyone).
3) most, if not all, OA applications are in reality revenue grabs (to take advantage of point one); or to try and find a spare path on paper that they know, in reality, won’t work on rails such that someone buys them off. Privately, many OAO people will admit this.
I don’t know Donny well enough, but suspect it is feasible without a new platform subject to signalling arrangements.