I don't know if it's a constant idea. But IIRC, Ottawa Confederation Line (O-Train Line 1) was initially scheduled to open in Nov 2018, but was postponed to Sept, 2019, far more than the six months I'm suggesting for the Crosstown. While, AIUI the primary reasons for the O-Train delay were related to technical issues and system testing,
winter weather also played a role, during the testing phase, with ice buildup on the tracks and issues with the train doors freezing, which required further adjustments and testing. So, it's not that transit cannot be opened in the winter (and, we have ready examples above of transit opening in winter), but it's easier to conduct final checks outside of the cold season. And there's also the issue of Torontonian's "transit delay fatigue" or "delay acceptance." After years of broken promises, Metrolinx and the TTC know we're not going to march on Queen's Park if it's delayed further for warmer weather.