The thickness of your line indeed determines your catch success due to a variety of reasons. It affects the way you cast, the bite sensitivity, etc. You need to find a balance between all the factors to come up with what line rating is most suitable for the type of fishing that you are doing. Personally, I can't imagine going beyond 15lb test as a main line and 10lb test as a leader when fishing in the Lower Mainland.
Using 15lb test line does not mean you cannot catch fish bigger than 15lb, it means you can't dead lift up an object that weighs more than 15lb because it'd snap. 12lb test main is what I use on the reels for drift fishing, the leader strength varies between 6 and 10lb, depending on the target species.
Here's a chinook salmon from several years ago caught with 8lb test Maxima Ultragreen running straight to the spinner.
This is more of an extreme example of course. It was actually quite surprising that it did not take too long to land this fish.
If I were someone who is just starting out salmon fishing in rivers, my concern would be figuring out what setup works best to improve my catch rate, not what setup can land me the largest salmon found in the river.