I think saying that LRT should never be built again is a pretty absolutist take, but what has happened with Line 6 is certainly worth being concerned about. I think we need to see how far they can get in fixing the issues on Line 6 over the next few months, but I largely agree that Line 5 and Line 10 are also most likely going to have a difficult time once they open. Ottawa's Confederation Line and Edmonton's Valley Line are the two best points of comparison and both have had a lot of issues.
My personal opinion here is that LRT is probably not the correct transit mode in most cases for what the general public expects to get out of billions of dollars in transit spending, as well as the future plans and expectations we are placing on the corridors. Speeds and reliability similar to those provided by the subway systems are what is expected from LRT in the public eye, and the governments treat these LRT systems like subway systems anyway when zoning changes are being made for extremely dense transit-oriented development along the lines. Surface LRTs obviously face significantly more technical challenges compared to subways, in that you have to deal with conflicts in signal priority, conflicts with road vehicles, and conflicts with weather. When you combine these technical challenges with long-term subway-like expectations, it is my opinion that LRT is inadequate, and will probably be overwhelmed long before their design lives are over. I think that smaller, less dense cities in our country can make a reasonable case for LRT (as with what has happened in Kitchener, and was on the table in London), but for a region like the GTA which is fast becoming very integrated, urban, and dense, the expectations that are being put on these systems warrants subways or winterized, fully grade-separated metro lines. It may be more expensive up-front and demand may not be there immediately, but I believe they will be useful for much longer than LRT.