From an urban planning perspective, there are some caveats to your points:
A new downtown would make a subway very easy and cheap to build, you could cut and cover instead of tunnelling
Cut-and-cover will make shallow underground tunnels cheaper to construct in almost all cases irrespective of building in an old city center or as part of building a new city center from scratch. In fact, older pre-WW2 cities are almost ideal for cut-and-cover because the tunnels can follow the street grid, yielding a tunnel which will be near to already-built destinations, while minimizing costly curves.
Probably the worst scenario for cut-and-cover is when the surface street has unnecessary curves and detours (eg American suburban arterials). So either the tunnel follows the curve and becomes weirdly farther from major destinations, or it’s built in segments using cut-and-cover where possible and digging for the rest.
Cheeeaaap land for huge offices, roads, and even houses
At least in America, where agricultural land at the edges of metropolitan areas is still cheap, the last 70 years do not suggest huge roads, huge offices, and huge house lead to a utopia. Instead, we just get car-dependency and sprawl, as well as dead shopping malls. The benefits of this accrued to the prior generations, who wheeled-and-dealed in speculative suburban house flipping, and saddled cities with sprawling infrastructure that the existing tax base cannot afford.
Green field is just so cheap.
It is, until it isn’t. Greenfield development “would be short term appealing but still expensive when it comes to building everything”. It’s a rare case in America where post-WW2 greenfield housing or commercial developments pay sufficient tax to maintain the municipal services those developments require.
Look at any one municipal utility and it becomes apparent that the costs scale by length or area, but the revenue scales by businesses/households. The math doesn’t suggest we need Singapore-levels of density, but constant sprawling expansion will put American cities on the brink of bankruptcy. As it stands, regressive property tax policies result in dense neighborhoods subsidizing sprawling neighborhood, but with nothing in return except more traffic and wastewater.
Either these cities must be permitted to somehow break away from their failed and costly suburban experiments, or the costs must be internalized upon greenfield development, which might not make it cheap anymore.
I think you’re spot on, since using a random power/torque/rpm calculator online, an 85 Nm mid-drive motor rotating at a reasonable crank cadence of 80 RPM yields a power output of 710 W. That’s just shy of the max power regulation in the USA for ebikes, at 750 W. And motors can definitely go faster than 80 RPM.
Mid-drive motors could probably be built with even more torque, but because electric motors maintain near-constant torque through most of their RPM range, the motor controller would have to perform more current limiting at the higher RPMs to stay under the legal power limits. So there’s less benefit, unless someone really badly wants more low-RPM torque.
At that point, though, other parts of the bicycle drivetrain might start disintegrating under such forces.