I find it amusing that people pretend the city never replaced lines before. Lines 1 and 2 used to be streetcars. Eglinton and Finch are being switched from busses. The TTC and city council botched the Scarborough RT Mark II upgrades which got the SRT switched to Subway TRs in a tunnel. The likelihood is very small that the Crosstown would be changed but it does exist. It is even smaller during the 30-year maintenance contract. But that is not forever.
The Crosstown might not switch to high floors. (As much as I wish they would) But it could be grade separated over time or as a big project. LA is incrementally grade-separating
parts of LRTs now. Melbourne elevated 8km as part of their level crossing removal project for their suburban 'metro' rail in a right of way that was narrower than Eglinton in Scarborough. They didn't even need to shut down the line while building literally on top of it
. Tokyo is a famous example of their lines being upgraded over time to be fully grade separated and high capacity. These things are possible. The only thing needed is civil and political will.
When the West Extention opens Eglinton Line 5 will be 19km grade-separated and 9km on street. That is more grade-separated kilometers than the length of the Ontario Line.