You'd be surprised at how little extra momentum (speed or weight) you have to add before disc brakes become essential. For me, the aha moment came when I started pulling a 40 lbs bike trailer - then you really don't want soft brakes when approaching a junction with cars. I installed some Avid Juicy's on 180 mm rotors and they're awesome - even now when the bike trailer is 70 lbs.
Disc brakes are catching on, but for good reason: they increase the stopping power in rain substantially.
Though, it doesn't matter if you don't ride in the rain, and many people don't.
They also don't bleed as much braking power when they heat up from repeated or long braking, which is more common at the high speeds seen in an electric bike.