FP+ should have been an upcharge to begin with! (And i say that loathing most, if not all, of the new WDW upcharge "events".)
FP worked because not everyone got them. I believe this is why Express Pass works, as well.
FP+ forces everyone to get 3. Now you have 80% of the queue devoted to these people, most of whom just picked one to fill out the 3. Touring Plans and EasyWDW have quite a few articles on the effect FP+ has had on standby lines. Spoiler: it made them ridiculously long.
Once they start charging for these, less people will be getting them, and it will barely make a dent on standby, like at UOR with EP.