How do you build transit infrastructure when you don't know where the demand is?

I encourage you to look into China's bullet train network, they did what you're suggesting. And the last I heard the system is struggling because the stations and lines weren't built where people actually needed them so it's heavily underutalized.

The most successful public transit systems were ones built up over time. It's going to take decades to fix public transit in many of our cities, are there any cities that aren't doing this?

Also remember that city policy falls under provincial jurisdiction. I was surprised this year to even see the feds start trying to throw money at that problem and incentivise cities to rethink zoning. But it takes time, and it also takes voting people who care into the right spots (city hall and provincial governments)