While I agree with most of what you said...since when is the line not past 50% design capacity yet? The line is seeing peak loads of over 12K PPHPD. Light rail, even in the grade-separated form, generally only sees up to 15K PPHPD.
Even with the existing trains, running them at 3-minute intervals only gives you a peak capacity of 14K PPHPD
This is directly after opening.
Ottawa had the highest per-capita transit ridership in Canada and the United States. There is no reason they shouldn't have invested the extra half-billion to billion dollars required to double that capacity with heavy rail. There's also no reason to believe that the City of Ottawa wouldn't be able to support a second rapid transit line through downtown within a decade or two. That being said, given the rollout of this light rail line, I have a feeling ridership will fall to a point in which that type of investment won't be justified for decades.
Even then, Peel region still has a huge transit user base (at least 100K go riders per weekday, the Hurontario LRT is supposed to carry 150K PPD just a few years after opening, and the bus networks in Brampton and Mississauga are quite extensive). There have been talks (albeit not that serious) of bringing the subway into Mississauga because it is a huge transit center with a lot of ridership potential.
But back to your original point: I completely agree. The lack of redundancy issue doesn't stem from the fact that there's no second subway downtown. Rather, the line that was built never had the redundancies built-in for increased passenger loading. In times of emergency (ie failures), there's never enough space for passengers, so delays ensue for far longer than they should, even when things get fixed. Over time, ridership increases, and with increases in ridership, come more capacity requirements.