If you look at transit mode share by destination location (rather than residence/origin location) in Toronto there's a huge difference between downtown and more outlying areas on the rapid transit system.
Transit mode share by destination (Toronto):
Central Etobicoke destinations have extremely low transit mode share, despite being served by the Bloor subway line, and I think a lot of this is due to it being fairly accessible by car. Transit mode share in most inner-suburban areas served by the subways is typically less than half as for downtown, and even in Willowdale (very dense + subway) it's only about half of downtown. Not that many people walk to downtown Toronto, less so in Willowdale. Driving to destinations in Willowdale is more than twice as common as to downtown destinations.
This is driver mode share by destination (GTA):
However, the differences between downtown and inner suburbs are much smaller if you look at transit mode share by location of residence (Toronto)
There's a lot of people in Willowdale commuting to Downtown Toronto. The area is more expensive than outlying areas - if you work in the suburbs, there's not as much of a point paying the premium for housing, a lot of people live there for the subway access to downtown. If you commute from Willowdale to Downtown, it really makes sense to take the subway, you save on downtown parking costs, and the subway is usually just as fast as driving (at least during peak hours).
However, if you live in Markham and commute to Willowdale, transit is much less advantageous. Even if your average highway speed is 60 km/h rather than 110 km/h due to congestion, that's still a lot faster than most forms of transit (when accounting for stops along the route, waiting for the bus/train, walking to/from stops). And parking in Willowdale is cheaper than in Downtown Toronto. So a lot more people drive.
Conclusion: I think it'll provide a lot of benefits, facilitate redevelopment of parking lots/garages in DTLA and allow new office buildings to be built without having to invest in as much parking. It'll also help support infill. But it won't lead to a huge paradigm shift in transit use, like I wouldn't expect NYC or Paris levels of ridership. But it could still lead to a decent increase in transit use (especially if new housing + employment is built near transit).
It's not just about whether there's a lot of rapid transit lines but where major employment centers are located. High transit ridership usually occurs when employment destinations are accessible from throughout the metro area thanks to the existence of many rapid transit lines, and when driving is inconvenient due to congestion, narrow streets with lots of traffic lights, high parking costs, etc.
Although people in LA like to complain about how terrible their traffic is, I suspect it's still not *that bad*. Yes, you'll be going a good bit slower than how fast you could be due to congestion. But that un-congested baseline speed is still quite high thanks to a lot of freeways and boulevards. Yeah, you're going to have a 90min+ commute in each direction if you try to commute from the periphery to the core, but LA is a huge city, there's plenty of housing options closer to the core, and plenty of employment options closer to the periphery.
Although most of the biggest employment destinations will be covered by these rapid transit lines, those still hold a relatively small proportion of overall jobs. And aside from DTLA, most of these employment centers are only served by 1-2 rapid transit lines, so it will only be convenient to take transit to them from parts of the metro area. How expensive does parking get in DTLA? That will also be a factor. And much of LA is not super dense. It's very evenly moderately dense, but it doesn't have the density peaks that you see along transit in NYC, Toronto or even Vancouver.