My concern with buying a disc brake bike now and planning to keep it for
possibly decades is that the technology is not mature yet. Rim brake
parts, as noted, will be available for pretty much the rest of my life.
But disc brakes may be still evolving to get rid of the weight penalty,
optimize rotor size, pad design, etc. Pretty much the only replacemetn
parts that rim brakes need are cables and pads.

Another concern, which may be unfounded, is that disc brakes stress
frames and wheels in much different ways than rim brakes do. I am not
sure that tecnhlogy has caught up with this yet. For example, on the
front wheel disc brakes can create a steering input during hard braking.
Braking forces are higher than acceleration forces, so this may affect
spoke and rim life.

In terms of performance, I have only ridden a handful of cable actuated
disc brakes. I found them abrupt and grabby and I didn't like them.
But I may not have ridden best examples of the technology and things
have probably come along since then (several years ago). My brazed on
centerpulls, cantilevers and single pivot brakes have consistently an
safely stopped me for decades and I have no complaint about them, but
maybe someone will design the rim brake that converts me. Those brazed
on Mafacs are wonderful, BTW. What a great brake, but few people have
had the opportunity to try them.