Worm. Open. Can.
Part of the reason now that so much more braking is available is that the traction from the tyres and the quality of suspension travel have now moved on a lot. If you have a rigid bike with old 1.9" tyres and cantilevers, because the tyres will constantly be out of contact with the ground (or at least not supplying max pressure, the brakes will easily be squandered due to the lack of grip available. If you compare that to a full sus xc bike with 4-5" front and rear, with the suspension set up to resist fork dive but still absorb most things without being too stiff then there will be far more grip available as the tyres are able to more closely follow the terrain. Add a set of slightly larger 2.1ish tyres on there which can be run slightly softer etc and there is a noteable increase in the traction available. This will help when cornering etc but also when braking (the same principle of suspension allowing you to climb over rougher tracks easier) as the suspension and tyres prevent the tyre leaving the ground, bcoming so unweighted and then losing grip.
I mean, don't get me wrong, the brakes on my big bike would be completely unecessary on my hardtail - there is simply too much power there and they also weigh a ton. For plain old riding a set of Magura racelines are superb when combined with a set of mavic rims - but they still need looking after. In contrast, the brakes on my xc bike were gracing the dh bike for 2 seasons abuse and aside from a hose damaged in a crash, they went untouched - aside from 3 or so pad changes a year. And you don't even want to see the state of my wheels on that thing. Which neatly brings me onto my next point. Rim brakes are fine when everything is smooth, the wheels are perfectly true, the brakes have no slop in them, the cables aren't gunked up and the rims aren't covered in slop. Mountain biking is, unlike road riding, mainly in the slop on this fair isle and that causes a problem. You get grinding, go through many sets of pads, have rub from when the rim gets a tiny bit out and you dare ride through a puddle (after which the brakes lose performance) and of course you also have the problem of running straight through the rims sidewall (unless they're ceramic). That then necessitates a new wheel. In contrast a rim brake will perhaps occasionally grind but because the disc is steel and the pads rock hard (on most systems), it causes no ill effects. There are only pistons which move and no pivots and therefore the slop is reduced and the element which is difficult to adjust and control on rim brakes. Yes, you do knock rotors but an adjustable and two minutes in the shed easily solves that.
Rim brakes are fine if you are a reasonable gentle rider but I would never consider them to be better than discs for anything other than reasons of retro-ness. A disc can be lighter, more reliable, more powerful and also more controllable.
One overriding memory I have of rim brakes is that there was a period after the pad touched the rim where bugger all happened (XTR V's on Ceramic rims) before things suddenly decided to grip and you would start to slow down (ok, exaggerated a little but you get my point). Discs give power and quite linear control from the moment the pad hits the disc - very much in line with car brakes.
I love this site, the retro kit is awesome and it really takes me back to when i started out as a kid but things have moved on at the front line for a very good reason - modern kit just works soo much better. No reason not to have the retro stuff mind - who said that sense had to come into the equation anywhere?!