I see this argument presented time and time again and it always bugs me. "Apple should spend less time on watch bands and more on laptops", etc. Yes, because it's clearly the same people working on both things. The people doing architectural designs for parking garages aren't the same people doing substation designs for rail electrification. Cutting back on one would do no good for the other. In fact, most of those design jobs are contracted out to consultants, so it's not even Metrolinx staff working on them, so there are no resources to shift other than money. There's an old saying that I think is apt here: "Just because one woman can make a baby in 9 months doesn't mean 9 women can make a baby in one month."
Yes, adding resources may very well speed up the process slightly. Have you checked Metrolinx' job board lately? Every time I look there's at least 1 or 2 postings for RER-related work.