On one level, I suspect they're on their way out as a trend, since they've been popular for 3 or 4 years now. The new styles of sandals for this summer have definitely been more minimal (I'm thinking specifically of ones from Madewell or J Crew, very hipster-normcore), with only a strap or two and less detailing. On the other hand, the ankle strap trend is still going, so the simpler gladiator styles are still reasonably current.
I do think a gladiator sandal is one of those styles that's always around during the summer, even if it's not the dominant trend. So if it goes with your style, I think it still works - just like even though Birks were not trendy for a while, I still saw people wearing them and it was fine, especially if they had a more boho/hippie/crunchy vibe going on.
To put it another way, I am still wearing my gladiators, but I don't plan on buying any new ones, and will probably be looking for a different style for next summer.
