Wide rims are fine if you run wide tires. Running wide tires slows you down on pavement, but is an advantage on loose gorund, so it's a depends on ones' use. A heavy rider does not have to run wide tires, and if he does he is just making things that much harder than they may need to be on pavement. Wide rims on narrow tires are a puncture problem. You can run the wider tires on the narrow rims but not the reverse.
One has to look at the real issue, this isn't a mater of what brakes are the best for a touring bike, but what brakes are best for a guy over 300 (adding the gear). I know a lot of guys who weigh that kind of number and are really into sports, yet the bike industry tubing is designed for guys around 150 tops on the charts. over that you are going to need to go beyoind road tubing. So when you have a situtation that is more that 2x stock, you have to think out of the box. I do think parts are a real problem particularly for the serious expedition guy or someone who cycles in areas with small shops, but again with a redundant system it isn't a big deal because if the disc blows, you still have the V or canti.
My first upgrade has been to move to the Paul brakes, however they are not all that special. What is special is the quality of the materials, no way to have a casting flaw. But the design is the same as many stock brakes out there, and in fact it puts bad loads on the canti braze ons, because the whole uint mounts about 3/8" further out there. I'm using mine now, but on my custom I am going to turn special boses, then I will have the best of both worlds, great brake and great mounts. However it is just a traditional canti, like from the 80s, it is not an eye popper like the first time one uses index shifting (whether you like it or not), just an incremental.
I am still considering the disc. The fact is I just spent like 400 canadian to redo the brakes on my f-150 . New pads, calipers, and wheels bearings. I put in the best parts I could get, and did the work myself. One could be just as banged up if the brakes on the bike let go, so I am spending some coin on my custom bike. I've got 2 pairs of Paul, but I am so far only running them on the front, I like my Petersen self-energizing brakes for the back, and I may add an avid disc to the fork. I am also thinking of a friction brake on a hugli hub. I tried out the Shimano gear hub the other day with the drum fitted and it stops the bike like you dropped anchor. I just like the idea of some redundancy both from the mechanical perspective and the conditions perspective, so no mater what, the bike stops. I don't have that on dual canti, and i have been running that set-up for over 20 years, so I do know how to adjust it, but the road levers just aren't as positive. And on the wide bars, I will have room for the 4 levers, the regular drops and one or 2 2-ingers style on the top.
On a motorcycle one caliper and disc of Paul quality, not to mention controls is 1000, I just don't see the point in going cheap, particularly on a custom.