This. There are some really good
YouTube deep dives into this topic. But what it basically comes down to is that ALL FastPass or Genie does is redistribute the crowd. With physical queues, people are standing in them, one at a time. You're in line for SM OR you're in line for Dumbo OR you're in line for Pirates. With FastPass/Genie, you're "in line" for one ride virtually and ALSO in a physical line for something else. Or you're wandering the streets waiting for your return time and making the pathways more crowded. Disney's idea, of course, was that you would be dining or shopping (spending money) while waiting for your return. But in reality, nobody eats or shops the amount it would take to account for all your virtual wait times, so instead you're pushed into smaller attraction queues, making them far longer than they would normally be. Plus everyone wants to be in a virtual queue for SOMETHING, so they're using FP/Genie for attractions that don't need it, artificially increasing wait times across the board. But the total number of people in the parks doesn't change.
Your anecdotal information about Universal is incorrect. On the contrary, it's far more controlled at Universal. They know day by day exactly how many people are in their deluxe hotel rooms/potentially using the free Express passes. And they tightly tailor the number of Express passes available for purchase accordingly. Because they're so expensive, it's incredibly rare for Express passes to sell out. But they do occasionally. When they do, it's absolutely possible that the Express waits will be 15-20 minutes, maybe even a touch longer.
BUT, you're not reserving that Express slot. You're not walking around for three hours waiting for a return time and THEN waiting 20 minutes. You're walking up to whatever attraction you want to do whenever you want to do it (except of course, for the handful of new attractions that aren't eligible for Express) and getting into that line--which is more like 5-10 minutes MOST days. Meanwhile, everyone who didn't buy Express sees an incredibly accurate wait time sign when they get in line. Because the number of Express passes sold each day is a knowable quantity, it's far easier for Team Members to learn the rhythm of letting in Express vs. standby and keep both lines moving.