On an electric bike, the motorization only needs gears if the motor power is limited. When you have enough power, you can drive the back wheel with a single gear ratio at any speed and on all terrain.
BUT if you are going to pedal the bike then you need gears to maintain a comfortable pedaling cadence at all speeds and on all terrains.
On low power e-bikes (roughly under 1500W) you normally have a single drivetrain that does both pedaling and motorization, so both share the same gears, which both help the motor and allow a comfortable pedaling cadence across all terrains and speeds.
On high power bikes (e-motorcycles), you have a single drivetrain for the motorization, and with enough power, there is no need for multiple gears, just a single gear ratio.
On mid power e-bikes (2kw-10kw) that also offer pedaling, you ideally find a dual drivetrain, one for the motorization with a single gear ratio, and one for the pedaling with traditional gear range for pedaling. This allows for simpler motors, easy throttle mode, and protects the pedaling gear system from the high power of the motor.
One last remark, combined motor an gears do not introduce any new technology. All of what is used in them has been done for decades and fully tested. The only thing new about them is the desire for them, which emerged from the use of E-bikes. This is why I am not worried about the technology itself, but only by its adoption, which is required for its ongoing availability and development.
