This line should have been a subway which means it could also have elevated sections and it should have had far fewer stations on the underground and especially at-grade sections. While the TTC and the City are responsible for the current slow running due to it's stupid vision-zero policy and it's lack of signal priority, the actually fundamental issues as to why the line took so incredible long to build is 100% on ML. ML couldn't have been more incompetent when designing this line if they tried. The issue is NOT about P3 projects which can work well {see Vancouver Canada Line} but rather how ML put it's plans out to tender and what those plans included. A 5 year old could have done a better job.
When you put a contract out to tender, it is exactly that, a contract and once signed, there is no going back. This means you need to put out a tender with 100% of what is expected........vehicles, construction zones, speed required thru sections, what the stations/stops will entail, what stations there are and where, trackage requirements, timelines,.............basically everything down to the colour of the paint. Once every single detail needed is determined THEN you put it out to tender and take the contractor who provide the best deal for what you need, the budget and timelines you have, with a reputable company that has a solid record of bringing in such projects.. Once the contract signed there are NO changes for any reason..........both parties signed on the dotted line and are expected to follow thru and if either party want changes during the construction, they are out of luck.
The juvenile delinquents at ML never understood this basic concept and the Eglinton fiasco is the result. ML wanted to change the rules mid-stream and then are somehow surprised that effects timelines, budgets, and doesn't result in endless lawsuits from the contractor. This is akin to so to going out and buying a car for a set price and then when the car is about to arrive, you want a different colour, different engine, and different interior design and expecting the car dealer to still provide the vehicle on the same schedule and budget and not expecting them to take you to court because you signed one thing but expect another free.