At scale we basically have 2 rail technologies.
Every technology is fundamentally different:
1. The subway is at the Toronto gauge with a specific width.
2. The RT is at Standard Guage (I believe) with LIMs, so the track is completely different
3. The streetcar network runs at the TTC gauge
4. The LRT lines run with the Standard Gauge
5. GO Transit utilizes the standard gauge and adheres to TCs heavy rail guidelines
6. The Ontario line would be basically a narrower subway with standard gauge.
As you can see, there's is an overlap of 3 particular types of trains: Heavy Rail Regional Rail, Subway/Metro, and Light Rail Vehicles. While one could argue that the SRT has its place as an ICTS, you could still make the argument that the line it serves could be more easily run with Light Rail or subway. We already have standards for subways in this city, use them. Stop trying to make every single new line unique, it makes systematic compatibility a lot harder in the long term.
Id say that it doesnt even matter how many rail technologies you have (to a point, obviously yes if you had 100 different technologies it will be a problem, quit hyperbolizing), whats more important is if those rail technologies are off-the-shelf and used around the world.
San Fran will have a harder time with BART than a city with 5 different technologies and trains that are used around the world.
Customization is a way bigger problem than fragmentation.
Having off the shelf LRT and Metro in whatever shape the Ontario Line takes will be better for Toronto than the current Toronto having the Mark 1 RT and custom CLRV/ALRVs
The TTC gauge isn't that different from the Standard gauge, as a result, rolling stock procurement costs aren't actually that different, if at all different. No subway train model for any city is "Off the shelf" by any means. Every system's track geometry is different, every line supplied voltage is different, a lot of track gauges are different, top speed requirements are different, seating layouts and platform heights are different, power collection systems are different, signalling systems are different, Tunnel geometry is different, door requirements are different, HVAC requirements are different, etc etc etc. The only real way you can save money on rolling stock is to purchase a lot of it at once. When you have many different lines with different technologies, you cannot purchase any significant amount of rolling stock at a given time, you cant interoperate them between lines (like when a fleet has a major problem), and training for mechanics, drivers, cleaners, etc is different. It just adds costs.
The costs may be justified if the OL was to have much less ridership than existing lines, but it's not. It's literally acting as a partial replacement of the Yonge line. There is no reason to change technologies for this line.