Flexible routes = unpredictable.

Flexible capacity = attempts at over optimization = not trusting that capacity will be there = unpredictable.

Shared infrastructure with roads and following road laws = all the inconvenience of traffic jams.

Flexible routes may be nice on a multi-year timetable but still runs into the jam issues above.

So if we are going to build dedicated lanes with special signal crossings and high capacity to encourage a large population to use it, let's make it as efficient as possible - a rail track is much more efficient than tires, and electricity is better than directly burning fuel, and delivering electricity through rail is better than battery.

Bus rapid transit exists, is much cheaper than rail, solves nearly all these problems while making the infrastructure far cheaper (due to ability to climb grade, etc). They’re often lighter than trains, they can be much more granular (so you don’t need an entire train at low rider volumes), so the efficiency can be closer to a wash than people would like. Especially with trolley buses or battery electric buses (or BEST, the combination ones).

It’s not an either/or situation. You have a bunch of tools with pros and cons and you have to use the proper tool where each one is most appropriate.

The best transportation systems are a blend of transportation types. Use rail where the benefits are worth the cost and buses where they are not. There also many incremental options in between.

Narrow buses (or long transit vans) in sewer tunnels. Get ‘er done.

Nah, that's for Teslas.

There’s a huge difference between good and bad metro lines. Just like how there’s a difference between good and bad bus systems.

Frequent and reliable metro with good coverage will beat bus system because of predictability. Also, capacity wise it’s much more effective, faster and etc. Unfortunately, most of North America’s systems are not of this type.

Buses are usually the cheaper inferior option. While bus service doesn't inherently suck, the type of decision making that chooses buses to minimize cost will usually cost optimize every portion of a bus network until the experience suffers. Bus stops without rain shelters or even a bench (versus stations). Dedicated lanes are not provided so buses get stuck in traffic. Operating budgets get cut and then the interval between buses goes from 15 min to 30 min. Service after 10 p.m. gets cut.

The bus networks that don't suck after usually called "shuttles" and are underwritten by tourism districts, airports, and theme park operators.

Buses can only hope to match the reliability and headway of metro services if they have dedicated right of way and signal priority for the complete route. This eliminates several of your supposed benefits.