Economies of scale is a distinct concept that's different than a flywheel. Yes, EoS plays an intimate role in powering a lot of flywheels. For example, the section of Amazon's flywheel that allows them to offer lower prices as a result of having more sellers works via EoS. However, it wouldn't make any sense to try to describe the entire flywheel concept using the term EoS, because lots of information would be lost, specifically the entire concept of a flywheel: that it feeds back into itself in a loop. Calling a flywheel EoS would be akin to calling a racetrack a curve.

As for whether or not "flywheel" is the best term, I think it's pretty good. Yes, real-life flywheels lose momentum due to friction, unless outside force is exerted. But the same is true with marketing flywheels, which is why all of them will have at least one step that requires the injection of outside energy, e.g. bringing in new customers.

That's still not a flywheel, that's a network effect. Like I said these are set terms from academia for these specific meanings.

Like I'd aid flywheels have always meant energy storage mechanisms that don't reach 100% efficiency.

Same as EoS, network effects can be a component of a flywheel, but aren't necessarily equivalent to the entire flywheel itself. Network effects simply mean that the value of the service increases the more people who use it, whereas a flywheel represents the entire positive feedback loop that may be driven by individual components like network effects, EoS, etc.

Marketers know very well what network effects and economies of scale are, and the concept of a flywheel is distinct from either. You can have a flywheel without EoS or network effects, and you can have EoS or network effects without having a flywheel.

Also, it's somewhat pointless to debate what the "original" meaning of flywheel is. Language evolves and changes, and people from one field borrow terminology from others when creating helpful analogies. Terms like "funnel" and "flywheel" are hardly the first to receive such treatment, and they won't be the last.