I'll just lay it out here that people who like to ride mellower stuff, like me these days, be the ones who lay out and design the B lines, instead of the people who have created a need for B lines in the first place. B lines -- I'd prefer the term intermediate or mellow intermediate -- should be incredibly interesting and flowy with fun but doable challenges and obstacles at a lower amplitude than a black diamond trail. If you'd like to ride an insanely well designed and super fun intermediate trail, head to Dead Horse Point State Park above Moab and ride the newer part of the Intrepid Trail system east of the park entrance road.
Stop trying to determine for everyone that there can only be one path in mountain biking -- advancing to and mastering increasingly more difficult challenges. Which is totally and completely legit for for some, maybe many, riders. However, I've been on more than one monthly ARR ride where a large group of riders just wanted to enjoy a fun, mellow ride, on mellow trails, away from traffic. They were super not interested in their ride resulting in a visit to the ER. Fun but interesting trails are good. The reason Walnut is so insanely popular is its high ride-ability quotient.
Also, something I've seen in play more than once -- throwing in something ridiculous that can be, let's face it, ridden only by a few super technically accomplished riders -- because the trail builders thought the trail was boring or too easy. Alternate lines will arise on trails that are erratic in their technical demands. If you're going to be building a black diamond trail full of technical challenge, fine. If you're going to be building intermediate trails, then do that with consistency. If you want to combine both, then have alternate lines and be done with it, because, as we've all seen over and over, riders WILL vote with their tires, then everybody clutches their pearls and takes to their fainting couch over the sheer awfulness of it all.
IMBA recommends stacked trails, with specifically black diamond trails further into the trail system, often as loops off a main route.