I take it that you're writing this from the Peterborough perspective? I ask because I'm writing from Niagara, another spoke where this might be the case.
As a rider, I'm far less concerned about who "takes the market", and far more concerned about how many places we can get to, how easily, and how often. This being the case, from your vantage point, how have things changed with a transition of service options from Greyhound to GO?
From my perspective, with the arrival of GO, my ability to access locations up the QEW through the Niagara Region and along the GO Lakeshore line has improved greatly. Travel to central Toronto has basically remained unchanged: I believe that the number of Coach Canada/Megabus departures now are similar to what they were 10 years ago, although it seems that the price has dropped relative to inflation. The private runs are usually busy, probably because they run express from the main population centres of Niagara Falls and St. Catharines: I save an hour, and often have a better drop off point in both Niagara and Toronto.
The glaring outlier in this story of general improvement is access to Hamiltion. CC/MB used to run regular buses along this route until 2 years ago.
http://www.thespec.com/news-story/4...ontinue-bus-service-from-hamilton-to-niagara/
According to CC/MB, it's the GO competition that pushed them out, although I don't completely buy the explanation. Whereas CC/MB traveled along Hwy 20 (traveling slowly but offering along the way) to central Hamilton, GO plods along the QEW and only reaches Stoney Creek or Burlington - both awkward trips away from downtown Hamilton or Mac. From my perspective, the introduction of GO as the main way to make the Niagara Falls-Hamilton trip has merely changed the trip from one type of bad to a different type of bad.
I imagine this observation has some similarities, but also some differences, to other locations in the larger network.