I think we're talking about two different things. On an e-bike generally a system voltage is chosen and the motor, controller and battery pack are sized accordingly. If you later swap out the pack you stay pretty close to the system voltage because the motor and controller are designed to it. Limiting power is done with current limit in the controller, which, at the power levels in e-bikes is usually done in software. Or in some cases like the Bafang BBSxx drives, by upgrading the FETs in the controller to hotrod it. A BMS acts at the cell or parallel group level, both of which are at a known voltage. If you swap the pack with new tech that has a different voltage per cell, changes must be made. Preferably in software.