First of all I love the idea, but every designer has his own journey and when I started working on Daily UI I explored more interfaces but like you even I was puzzled with lack of brief and came up with solution of ficitional breif.
While for me I lost touch after few days, as creating a breif everyday is very tiresome. And for others I presume their focus shifts on making better visuals.
I wouldn’t say Fix-UI is one top ultimate solution but definitely an alternative for designers would like to improvise their problem solving skills s. And as many commentators pointed out, without company study its useless but I feel we are going way ahead many of company need basic fixes like content readability or to not to get lost when you open home page, consistent design language etc.
I feel as designers we should have open mind and experiment with all and not doing things cause its not “one ideal solution” shouldn't stop us.