I don't care for rails on my forearm. If I can mount what I want on my rifle without rails, I'd be happy. I do want the advantage of a free float tube, but most have too large a diameter. I also prefer the protection from heat carbon fiber offers. I like how the PRI Delta carbon fiber forearm fits my hand so that's what I went with although it's pricey.
A move away from quad rails a fad? Unlikely. I think it's more of a practical choice as I feel quad rails are a specialized piece of equipment. Most don't need that much rail estate