Often, the opposite of a fad is described as a classic (or, in its adjective form, a classic [whatever]). This usage is especially common in fashion, but I think can be applied in other industries. The general definition, from Oxford Dictionaries:
Judged over a period of time to be of the highest quality and outstanding of its kind.
1.1. (of a garment or design) of a simple, elegant style not greatly subject to changes in fashion.
A work of art of recognized and established value.
1.1 A garment of a simple, elegant, and long-lasting style.
And some further explanation of the fad-trend-classic gamut, from the world of fashion (in this case, jewelry, but very similar discussions are found in other sub-genres of fashion; all bolding added):
Typically, fads last for a total of one season, but they can also last less than a month.
. . .
Trends have a much longer lifespan than fads. . . . The primary difference between a trend and a fad is that trends have the potential to be long-term influencers on the market.
. . .
Classics are forever.
"Fad, Trend, or Classic: What's the Difference?", MarkSchneiderDesign.com, 2014
While you will (most?) often see these distinctions discussed in the realm of apparel, they are also very often used in other areas.
Some relevant examples:
Note that it's really hard to judge something a "classic" before at least some time has passed, even if we think it is headed in that direction. So if you are wondering whether fidget spinners are going to be here-and-gone or will be around for a very long time, you could say:
Are fidget spinners just a fad or are they destined to be a classic (toy)?
A slightly more concise way to say this would be to use the term "new classic", a slightly-oxymoronic phrase meant to suggest that some new thing has all the hallmarks of a classic. So:
Are fidget spinners just a fad, or are they a new classic?
I note that another answer suggesting "classic" has been posted while I've been writing this; I'm going to go ahead and post it anyway for its examples, and also upvote that answer.