Most people here are avid transit users who care more about transit they can use and ride on rather than "ooh shiny impressive trains". Compared to many other places on the internet, this is arguably the least foamery transit forum I have seen. Most people want to have a serious discussion on what works, instead of "elevated because elevated is cool".
Because North American Urban Planners DEFINITELY do not have a long and storied history of making boneheaded decisions based off some idealistic goal that happens to be popular at that point in time, and developing severe Myopia when it comes to decision making...
Debatable. If it was fully grade separated, they could run the trains at significantly higher frequencies, which means while getting to the platform takes longer, on average they will have to wait for the next train for a lot less time whilst on the platform, and the actual travel time that is gained by not being on street would also significantly help offset the time (also if they only need to travel a few blocks, that's what parallel bus service is for).
Finally its important to look at Eglinton as whole. The line will be absolutely massive once fully built out, connecting to several GO, subway, and other rapid transit services (Line 1, 2, Kitchener, Barrie, Stouffville, LSE, SDBRT, Mississauga Transitway, and potentially much more), goes through all 6 of the former boroughs, the biggest airport in the region, a massive university campus, all while being on a corridor with massive development potential. All of this screams "Perfect for a Line 2 supplement".
This line is as much of a ML problem as it is a TTC problem, if not moreso. Boneheaded decisions like putting the line in the median of Eglinton east of Brentcliffe was 90% on Metrolinx, and a lot of aspects of the line such as surface stop design were butchered and neutered once Metrolinx got a hold of the project.