Let's be absolutely clear here. What you see here is not a concrete service plan, and should not be taken as gospel. What we will see running on this line in 2030 will almost certainly be completely different, based off completely different assumptions. In fact I'm willing to bet that Clarkson will end up actually being a local stop, with all trains stopping at Port Credit instead (this is pure speculation however). Based off what we know, it will likely be a lot more frequent than this (with some numbers Metrolinx provided implying up to 5-6m off peak headways on some lines). This is compounded by DB's plan to not purchase EMUs, and instead solely purchasing Electric Locomotives, and matching the speed of EMUs by running smaller, but more frequent trains.
The points that you need to take away from this are:
A) There will be express services for those who need fast regional travel
B) We will be seeing a massive increase in headways. I wouldn't be surprised if those hourly and half hourly trains run every 10m or so. So no, especially on LSW and Kitchener, building new stations isn't going to slow down service for most people.