GO's ridership kinda falls off a cliff once you get passed the inner-905 stations. Only a few hundred people board at stations like Guelph or Barrie. It's not entirely clear these non-commuter GO services really make much sense by themselves.
It does matter that these cities are not core-centric. It leads to really significant last-mile problems. Take the proposed Peterborough railway; lots of Torontonians have cottages in the region, so you'd maybe imagine the train being a good alternative to driving. It's a terrible alternative though since, even if it was faster, you'd then be faced with >1hr getting from the station to your cottage!
There are ways you could offset this, but regional rail (like flying..) faces big access time issues given how sprawl-y most Ontario cities are. To really be faster than driving, inclusive of access times, we'd have to start seeing real HSR speeds (~200km/h) on most rail lines. To do this on any large scale though you'd have to spend tens of billions across Southern Ontario (electrification, new EMUs, track work ect...) as a starting point. And sure, it would be neat imagining a hub high-speed rail network centred on Toronto. Would it really be that much better than the alternative though to justify the huge investment?
Let's say GO got its ass in gear and introduced a 200km/h train from Kitchener to Union Station, bringing travel times to 30-40minutes. But, for the sake of argument, let's also assume a bulk of riders are coming from UofW or WLU. So, getting to the train is about 30 minutes on public transit, plus some cushion time to make sure you don't miss the train. The one way trip is now 60-70 minutes. If there were simply a bus from campus which ran express to downtown it'd probably take 80-90minutes. Still slower, but not necessarily by much considering option one would need billions in capital work. If you consider more disaggregated travel patterns, say UofW to Markham or Scarborough, a direct mini-bus would almost surely be faster than the rail option!
Ehh, I guess it's just semantics, but I think of "regional" transit as something more like Toronto-London or Toronto-Cambridge. Trips between city pairs which don't have significant daily commuter volumes. Obviously there can be overlap between commuter and regional rail, but routes which can't take advantage of the Toronto commuter market probably won't be economical on their own.
Most increases in GO ridership have been coming from long established stations like Pickering or Oakville, not the expansions to Barrie or Waterloo. This is where the distinction between regional and commuter rail becomes significant.