It typically takes years to ramp up a park's attendance (granted that's based on past models and there's no recent comparable example among major US parks). I'm not sure what Universal's expectations are, but I'd expect around 7-8m (annualized) visitors in the park's first year and then 3-5 year ramp up to a target around 10m+ per year as you build word of mouth/advertising/repeat visitors over time.
Just feel like they shouldn't risk competing with Disney's 50th when a new park launch on this scale is something of a 3-5 year launch, not something you try to win all in year 1.
You make a fair point though that 2022 will probably be as busy a year for Orlando as there's ever been, so that's a factor in favor of 2022 but I'm not sure if a new park needs that to support it.