regarding brake power: With the rim brakes (2008+ version) I only once suffered from unsufficient braking and that was when going a very (!) steep downhill at speed, fully loaded and heavy T-bag on the front and with a 30+kg trailer attached. In this situation it was not possible to come to a standstill adequately. Once, in roughly oneandahalf decades of almost daily use of Bromptons.
No doubt disc brakes are fine and do offer more break power - but it is in everyday use just not necessary on a Brompton. Possibly the cry for disc brakes is once more the result from old messaging: Before the MK3 was invented in 2000 the Brompton brakes were truly terrible. Starting from 2000 this enhanced massively. In 2000 the front caliper was changed to DualPivot, the rear followed in late 2007. Next the brake cables changed to Jagwire and finally the levers changed to todays shape in 2013 (which in itself was a massive improvement). I do consider the brakes absolutely sufficient, even more if you use something like Koolstop salmon or SiwssStop Blue brake pads. When tuning levers, cables and pads even pre-2000 Bromptons learn how to break properly with their wonky calipers (including the red entry-level C-model that used the old-style calipers until end of production in 2007).
Bikes in Europe have to fulfill various ISO-norms and one of them is about the brake power. The Brompton complies to these norms, thus one can assume that the level of brake power is officially considered sufficient. On the other hand: Over the years Brompton have I think lowered the max weight of rider and luggage by something like 20kg if I remember correctly. This may have had to do with those norms for breaking.
Regarding disk brakes: If something is not properly adjusted with them they can drive you nuts. Especially with things like squeezing in the wet.
Rim wear with rim brakes on the other hand is there, but often vastly overestimated. A couple of years ago I asked around a couple of long term Brompton riders and it turned out that most of them had to change them around the 20k km mark. Some had far higher values, some way lower ones. It obviously depends a lot from riding style, riding conditions and rider weight. Also, Brompton have changed the rim models a couple of times over the years. Still I'd say a min of 10k km is a pretty safe bet for most riders. Still there are some who have to change earlier, but anything in the world can be broken when treated improperly.