Disc Brakes were available on several models of otherwise ordinary bicycles in the early Seventies.
Some of the Schwinn kids bikes in the stingray like style had them.
Heck, even JC Penney's marketed a line of 10 speeds with disc brakes!
Yes, absolutely, the disc brakes provided much better stopping power than what any other bicycle without them no matter the cost/prestige marque!!
Yes, that is true!
They added a heck of lot of weight, relatively speaking to lightweight bicycles that were striving to become lightweight for the consumers that wanted to reduce weight in grams in any way possible.
The other issue with respect to disc brakes at that time was that the replacement parts channel was limited to maybe the Schwinn store for their bikes with them, or somebody in timbuk2. Many folks already knew how to service and adjust the ordinary caliper brakes, and pads and cables, etc were easily sourced and widely available. It was very simple.
Folks ridiculed those 10 speeds in the 1970's that were disc brake equipped. Added weight was the main factor but also added complexity and unfamiliarity with issues when said disc brakes were not properly adjusted.
Disc brakes aren't a new concept. They improved braking significantly in the 1970's but were not widely adopted as everybody wanted lightweight everything. Today, relative to the 1970's, bicycles are much lighter across the road-bike spectrum so there isn't much an issue with weight today.
I am sure that todays disc brakes are lighter in weight and more refined but the fact remains that the early disc brakes of the seventies did a damn good job of stopping.
Were they needed then or today? Look at it this way, if you're hauling azz, you've got to anticipate your stops when on a bicycle or a motorcycle.......as it is not the same as being seatbelted inside a car which has four wheels on the ground............you can only remain under control at certain limits while extreme braking on a motorcycle or a bicycle.....
You brake too fast and too hard, and the human rider (YOU) will possibly become a soaring projectile, much like a football that has been kicked on a field goal try.
As was mentioned by someone previously, many features are marketed as the most essential, greatest thing since sliced bread. Certainly in some instances for certain bicycle useage, disc brakes might be more beneficial but largely there is probably not a material difference in stopping while cruising along at 11mph or 13mph on a flat bike path, or park paved roadway that is smooth and mostly without hills. This of course assumes that the ordinary brakes are in good order with acceptable condition pads and properly adjusted brake cables.
Any merchant such as Wallyworld or the world's better bike manufacturers would consider anything if it were both cost effective and could hype and booster unit sales. This isn't limited to just the low cost box store econo bikes, as you see oddball gimmicks from time to time at nearly every level. Yes, the better more prestigeous makers are definitely a bit more careful with crazy oddball gimmicks because if the public opinion turns more negative, they have more to lose because they have a prestigeous established reputation of quality that can be seriously tarnished while the low priced, box store bicycles won't lose such respectability across the board. (because they don't have any that they could lose!) People view the box store offerings on a singular case by case, individual bicycle model basis. They largely assume that said Wally bikes are at best, decent for the money and on average a crapshoot where some are awful and some are borderline decent if properly assembled which rarely happens at your Target/Dicks/Wallyworld etc.
If any Wallyworld bike gets decent reviews and amasses large sales over a period of months and years, that bike becomes sort of famous among the inexpensive bicycle offerings of the world. Walmart does have a few of these, which help to bring back bicycle shoppers year after year despite that a few other models that they also carry aren't so great. Not everyone needs an expensive bicycle. Some of Walmart's offerings are perfect basic bicycles at the perfect pricepoint for just how the buyer plans to ride them. Who cares if they weigh 30 pounds or if they are available in only two sizes, one for men and one for women..........or if in some models that they come in just one size, IF THE RIDER CAN COMFORTABLY & SAFELY RIDE SUCH BICYCLE after making seat height adjustment...............
Walmart fills the needs of a great many folks who want to ride a bicycle. Serious "cyclists" and local bike shop owners might dislike that fact. Just listen to the topic of this thread........"CONSUMER GRADE", as in that those that ride bicycles do it differently than the "cyclists"......(Sure some "cyclists" really ride getting to use all the benefits of weight/technology that their bike provides, while others are riding like a 94 year old grandma driving a Ferrari to church one mile away once a week and occasionally to the liquor store, the grocery store, the beauty parlor, and to wednesday night Bingo, which are all within 2 miles from her home.)
One could even make a case that buying a consumer grade Walmart bicycle for approx $200 or less, and then disposing of said bicycle when it needs repair work other than tires/tubes, brake pads etc,, ....disposing of and buying a new approx $200 or less Wallyworld bike is a cost effective strategy for most people!! They don't need anything state of the art or fancy and the bottom of the line new offerings at the local bike shop won't be any better for them at three times the price!