I might be drawn and quartered for saying this but, frankly, if building a Queen subway (and, believe me, a subway would be economically successful from day one) means the death of the 501, then - as Leslie Feist might say - "let it die".
I am a transit enthusiast but not a nostalgist. Transit should be about moving as many people as quickly and comfortably as possible.
But you are penalizing the wrong people/service. Streetcars provide a specific service that subways do not...it isn't all just about the fuzzy nostalgia and comforting feeling streetcars bring to riders or the contribution they bring to a neighborhood (not to downplay the importance of those things). Replacing a streetcar with a subway is not an improvement...it's losing one and gaining another...a compromise....a trade-off. And not a good one considering the important role the streetcar has had on Queen for well over a century.
It isn't about streetcars vs subways...on Queen, it's streetcars vs cars. And when we have one struggling but important transit route vs just another one of a zillion congested roads...then the solution is clear... the car should definitely lose when up against a streetcar route when the two cannot perform reasonably together.
The status quo simply means everybody loses...streetcars and cars (it's no party for cyclists on Queen either).
Taking a very busy, fairly narrow high street, and expecting high capacity frequent rail service, sharing lanes with mixed traffic, complete with street parking and left turning to work is just plain silly.
The only viable solution is simply a tough pill to swallow, which is why nobody has the balls to make it happen. First of all, left turning on
any streetcar route should
never be allowed...period. The convenience factor for cars vs huge problem for transit should make that one a no-brainer.
Short of turning the problematic sections of Queen into pure transit/ped-ways, it will simply have to put in ROWs for the streetcars with fully functional signal priority at all lights, with zero left-turning allowed. Now that transit has been given what it needs to function properly, then they can compromise on the space that's left for car traffic and parking.
Cars don't like it...too bad...there's plenty of streets purely devoted to cars to choose from.
As for mass-transit cross-town subway routes...sure, I suppose we could use one below Bloor...build one...it just has nothing to do with the 501.