The main disadvantage of rim brake on a small wheel bike isn't the lack of braking power (there are excellent rim brake calipers and V-brakes) but the rim wear which on a small wheel bike is much faster than on a big wheel bike. Rain and dirt accelerate the rim wear.
About the availability of rim brakes parts, I do not think there is any risk and obsolescence also affect disc brakes, not that disc brakes will disappear but current ones will be replaced by other models with other pads and even other mounting.