I'm not on here very much any more and this may have been beaten to death, but in case it hasn't been --
Rather than an alternative to the Line 1 extension, wouldn't the RH line be better renovated as (a) the northern half of the Ontario line, with a smooth interchange at Eglinton, provided (b) much better physical, fare, and local traffic-sink integration at Finch (OC-39; Newnham) and Sheppard (Oriole-Leslie; NY General)?
That sidesteps the wonkiness from Eglinton on south and maintains high-capacity north-south transit connecting to some of the city's busier routes. And how much slower would the Ontario line be at completing the southern half of the trip, for those going all the way to Union -- a diminishing number as the city becomes more polycentric, I have to assume?
(That also assumes some things about the ability to do physical and fare integration, I know, but the alternatives seem so much worse in terms of stranded infrastructure.)