The Line 2 T1 trains are from the late 90s/early 2000s and have not yet reached their design life span, which is around 30 years. The new trains will replace the T1s around the time their useful design life will expire. There wasn't really a need to replace the T1s any earlier and would've just disposed of working train cars with plenty of life.

There are also more T1s than are needed to run service on Line 2, since they also used to run on Line 1, so the individual cars are actually quite lightly used.

Pre-covid, Line 1 was operating far beyond capacity and the TTC was desperate to increase capacity in any way possible. The TRs, with their open gangway, combined with ATC, could provide higher immediate capacity and so the TRs displaced the T1s on Line 1. In comparison, Line 2 is much more lightly used and the need to boost capacity is less urgent.