Well there are always buyers at a price, think of pricing as a voting mechanism where people are voting at what prices they are willing to buy, the market then finds its price from this.
Though to say it keep sliding is way off the truth, it has been growing for 9 years and is up +1,000% in a year.
Backed by nothing is also not true. All currency has different inherent value and many of the Crypto tokens which support services derive value from the cost of the service.
RE: Daily Crypto Update — Unsteady Crypto Market (5th Sep ‘17)