Swept up in a swarm of concertgoers inching along a pedestrian bridge out of Sphere Las Vegas , there was nothing to do but join in the rollicking “I Want It That Way” singalong. The minute that opening night of the Backstreet Boys ’ Into the Millennium spectacle ended, it was like Sphere unleashed a roaring wave of white-outfitted fans, which gathered speed and decibels before crashing into The Venetian Resort, where “Larger Than Life” blared across the casino and two pals singing BSB outside the restrooms promptly evolved into a crowd of hundreds. “Vegas is dying,” TikTokers meanwhile declared as reports showed the town’s biggest visitor decline since the pandemic. “Vegas is dead,” others insisted on Instagram. “Well,” I thought, while riding the wave of white back to The Venetian. “If Vegas is dead, at least it’s spawned my kind of ‘90s heaven. There’s even a heavenly dress code.” “It was funny seeing those, ‘Vegas is dead,’ posts because we went, ‘Did you film that at 4:30 a.m.?…