My concern is as such, because while I agree that driving density is a strong hypothetical use case for subway construction (a talking point I often parrot), I am looking at this through 20/20 hindsight and through knowing the politics of the 2000s. Due to amalgamation and the shifting priority of the city council, Sheppard never got the density it needed to sustain a subway network, nor did it ever get the necessary extensions to make it a reasonable line. It was built, had poor ridership, then we spent 15 years arguing over whether or not it should be extended as an LRT or a Subway.
Eglinton would've been the same story but even worse. Eglinton back then was a lower ridership corridor than Sheppard, and unlike Sheppard which at least terminated at a mall, Eglinton West terminated in the middle of nowhere. Furthermore iirc, Eglinton never even had a planned extension to Yonge, from the beginning it was planned exclusively as a line that went from Allen Road to the airport, so we can basically forget about serving Midtown Toronto and Golden Mile. Best Case Scenerio, we would've built a linear transfer LRT to the airport to the west. Worst case scenerio, we did nothing with the line as we repeat the history of the Sheppard Line as a stub to nowhere, and construction on theoretical east and west extension maybe would've started in the last couple of years. I'd honestly take the poorly thought out LRT we have now vs any of those alternate universe Eglintons.