I agree that residents of Milton should be able to get downtown with ease, but as someone who uses the route on a quasi-regular basis, I have found the current arrangement, mostly, to be more convenient.
It took some doing. When they first rerouted the 21 to Oakville, there was no midday service along Derry Road and it took them eons to wake up and run all the 27s to Milton, but now that all of them do so, it is a marked improvement. The biggest problems with the current 21 in Oakville is that it doesn't run during rush hours (which is a complete joke), and that it runs every hour for most of the day (30 minutes would be much better). Transferring in Oakville was fine, there's normally a train along within 10 minutes or so, but the reverse direction is brutal, sometimes you could be stuck waiting an hour for the next bus. So of course, instead of working at these issues, Metrolinx are reverting the changes and throwing us into Gardiner traffic.
If you gave custody of transit in Ontario to a bunch of strung out crackheads, I doubt very much they'd do any worse than ML does.