What was it that caused all the momentum and hype from the mid-2010s to vanish, I wonder?
Much of Dubai's boom was built on hype that wasn't going to materialize anyway. The big hype period wasn't in the mid-2010s, it was in the early 2000s. "Everybody" wanted in on Dubai's property market, and skyscrapers were built with insane profits. Put figuratively, one could stand in line to buy an apartment, get the documents, then head to the back of the line to make a fortune selling it on to the people standing there. Then the global financial crisis hit Dubai, and most of the big projects were put on hold. Some of them still haven't continued construction, with the half-built ruins standing in the desert for over a decade. A lot of big developers folded up and many lost a lot of money.
The announcements in the mid-2010s were mostly an attempt to bring back the boom, but it didn't quite work. The belief in Dubai's economy just wasn't there, and it hadn't got as many tourists as projected either.
@shawnoc summed up the core issues pretty well, but there have also been other factors. Much of the tourist spending in Dubai comes from neighbouring countries, and they aren't doing so hot right now. Russians were another big group investing in Dubai, but that economy is in the toilet after the war in Ukraine and ensuing sanctions. Egypt had its revolution and ensuing economic turmoil. Syria, you know the story. Pakistan is also going through a rough patch. There's also an aspect of aviation, of all things, to consider. The UAE government spent big money building DXB up as a major hub airport, right before new planes such as the A350 and 787 Dreamliner made the hub model for airports somewhat obsolete. Instead of flying from A to B, change planes, fly from B to C, airlines just set up a direct route from A to C instead. Suddenly Dubai became flyover territory instead of the place everybody stops on the way from Europe to Asia. So yeah, nobody is placing money in Dubai at the moment, at least not to the degree that everyone was expecting.
And I have a feeling the big crash hasn't come quite yet either. Many new residential skyscrapers are still built in Dubai, as if everybody is expecting the conditions of 2005 to come back and want to have as many apartments on the market as possible when that happens. Apartments, malls, and offices are still being built at a breakneck pace, but fewer and fewer of them are being sold. The construction industry makes up a good 20 % of Dubai's economy, with the property market being another 20. Right now, it's in a phase where it can't stop building, because then the construction industry would collapse and take the banks with them, but the developers who pay the construction industry aren't making money, so the situation isn't sustainable. The dominoes are set up to fall, and it could get pretty ugly. I wouldn't expect many good news out of Dubai's tourism industry for another few years, at least.