Ok I am going to have to push back on this as it wasn't the streetcar network that prevented the Queen Street line from being built, it has always been stalled by politics. Firstly in 1910 after residents voted in favour of Streetcar subways under Yonge and Queen, only for that proposal to be cancelled after the next election due to its cost. Also this first proposal wasn't just about "rapid transit" but also getting around the Toronto Railway Company's ownership of the network. Once the city took over the network in 1921 this stopped being an issue. The next proposal came around in the 40's after WWII with a heavy-rail subway under Yonge and a Streetcar subway under Queen. This was derailed because the Province and Feds couldn't agree on a post-war employment program so only the Yonge Line could be built at the time. The next proposal came around in the 50's/60's but this one fell through because the Bloor-Danforth corridor had become the city's primary east-west corridor and it is where all of the suburban riders from Scarborough, Etobicoke, East York, and York were being funnelled on to. The BLOOR streetcar was way over capacity (something like 18,000pph) and replacing it was now the higher priority. So while politics didn't kill this proposal the city's suburbanization and changing travel patterns did, and the suburbs would continue to be the biggest obstacle for constructing the line moving forward. In 1967 the first wave of amalgamations would see the Villages of Mimico, New Toronto, and Long Branch wiped off the map and with them their support for the Queen Streetcar Subway, leaving Toronto as the lone voice on Metro Council still championing the project. Since those three villages were streetcar suburbs anchored around the LONG BRANCH Streetcar they supported the Queen Line since it would speed up travel times for the streetcar, especially if it was connected to the Queensway ROW. From here on the balance of power on Metro Council shifted to the suburbs and every transit expansion project would be out there as it was both cheaper and more politically popular with suburbanites to extend the subway further into the burbs then build a brand new line downtown. As well with the three villages out of the picture there was no point keeping the line as a streetcar subway, so shortly after this the line evolved into a heavy-rail line. The DRL would pop up again in Network 2011 but that fell through after Davis retired, and the Liberals had no serious plan, and the NDP couldn't afford to build anything (iirc the DRL wasn't even part of the NDP's plan).
So it wasn't the legacy network that stopped the Queen Subway from being built as the city tried multiple times to build it. The blame for this lay largely with the suburbian municipalities which were more valuable to politicans then voters in the inner core. I remember when Rob Ford was making rediculous statments about how "Downtown had enough subways" eventhough not a singe new subway station had opened downtown since 1966. While at the time he was making those statements the Sheppard Line was only 10 years old. Finally the precedent of replacing streetcar lines with subways had already been set in this city as we did exactly that on Yonge Street and Bloor and Danforth, so I don't know how the legacy network prevented us from building a line along Queen Street when we had already replaced 2 other streetcar lines. Ideally we would have built the Queen Line after the Bloor-Danforth, now of course what form the line takes be it a streetcar subway or heavy-rail subway is a matter of debate but as we all know, that is not what happened. Instead the subway was expanded further in Scarborough, Etobicoke, and North York as they were now the ones with the largest voice on Metro Council and the rest is history.