You dont do engine break-in to make the engine more powerful though. You do it so the moving components would mesh with each others small imperfections, would get a surface oil layer etc.
Cycles would probably be better, but how is constant rpm bad for the engine?
I would compare it more to sanding a wood block, doesnt matter how fast you do it, you get to the same end result.
I mean I would most likely take it easy as well at first if I got a new car, but I assume some level of it is done during the car assembly and a certain degree of it comes from "better safe than sorry" and "engines have always been broken in"
The gym example was a point about stagnation, not power. Because constant RPMs keeps the internals 'cool', you're not working the motor out to bed everything in. You need cycles to break the engine in, and only heat will create proper sealing properties.
Yes, engines are ran at a factory dyno and 'broken in' - but they are done so without stress. There's no load put on the engine, no parasitic losses, no 4000lb shell to haul around, nothing. It's just a guy in a booth that opens the throttle a few times, and approves the motor.
That's why we still have the break-in period. Because now that motor is under actual duress and it needs to be taken care of for a little bit. If engineers require it, we aren't the ones to be questioning it.
Edit: Also, here's an actual engineer's explanation (Engineering Explained). There's a very frequent highlighting of varying load on the engine:
https://www.youtube.com/watch?v=oklqJnm7_TY&t=367s
Bottomline: Don't drive at constant speeds too long in a new car. The motor needs to be exercised at varying loads until it can be fully opened up.
