Things could obviously be built after Transit City, but it was definitely meant to be the be-all and end-all of transit construction as far as David Miller was concerned. We're talking about a long-term transit plan. It was already Miller's second term, and it would be unrealistic to think he would have won re-election repeatedly. I'm not convinced he would have defeated Rob in 2010. But let's assume an extended Miller mayoralty...
Adam Giambrone said a DRL would not be considered until Transit City was finished in 2018 (as if that date was realistic!). But who is to say they would support the DRL in alternate universe 2018? After all, it was already rejected once as there was no DRL in Transit City, which still boggles the mind.
There was one occasion when council under Miller had a position on the DRL In response to the province's support for the Yonge extension in January, 2009, Miller agreed to have the city pay for a Yonge Environmental Assessment in exchange for Metrolinx moving the DRL from its 25-year plan to its 15-year plan. That was pretty much the extent of his interest in the DRL. So, presuming Miller stayed on longer as mayor, the notion that other projects would have started concurrent with Transit City is far-fetched.
As for Scarborough having their RT replacement, maybe, but why wasn't the LRT included in the first wave of projects? That should have been a no-brainer. The RT replacement and the DRL were, by far, the city's two highest transit priorities, and Miller addressed neither. If he had, transit would be far less contentious today. Instead, he made political choices prioritizing Sheppard and Eglinton.
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As for the current situation, with the Corona crisis it seems likely at least one of Doug's big four (Ontario, Yonge, Scarborough, and EWLRT) will be chopped, or more likely delayed indefinitely. Yonge for me is easily the safest of them all. It has a completed EA, is closest to shovel-ready, and may make Doug more competitive in York Region. Scarborough was Rob's last moment of glory as mayor amid a fog of a gazillion scandals. Can't see Doug pissing on Rob's memory. EWLRT runs through Doug's neighbourhood, and he won't want to risk a future premier building something other than an underground extension. So I'm betting on Ontario. It's the most expensive one by far, needs federal support, and provincial Liberals are apparently lobbying their federal cousins to not fund it unless Ford reverts back to the TTC plan or at least hold off and wait to see if Ford loses in 2022. Not to mention that Doug will likely win zero seats in Ontario Line country.
Coming soon to Yonge: massive crush loads.