Milton is not the only GO service in Mississauga though, so I don't see how this is valid as a 1:1 comparison.
I don't know what GO is expected to do; we live in a country that allows freight operators to hold passenger trains operating on their network hostage, this is the result. Good luck trying to change that. The only thing that can be done is to either expand the corridor, at great cost, to give GO their own tracks, or build a bypass for CP so that they exit the Galt sub. Both would be incredibly expensive, long term projects. If superior service can be achieved to Kitchener at cheaper cost in the interim, are they supposed to not do so, out of respect to the people who live along the Milton line?
As I keep saying - do yourself a favour and get a car. No matter how much we want to pretend to be supporters of transit, there are many use cases where that won't be a feasible option for decades to come. Rejoice that the option of tearing down the Gardiner has been taken away from Toronto. Small victories.