For me 3 days it's perfect for Universal but with
Epic Universe will need to add probably 2 additional days depending on how we feel about repeat stuff at the older parks we might be able to do just 4 (2 for the older, 2 for the newer).
However, if you frequent the Universal side of the Boards many of them spend longer times at Universal, 5-7 days being more normal.
During the pandemic it did seem to push people away from Disney to Universal, less split stays but rather abandoning Disney trips in favor of Universal because of the overly complicated and expensive aspect to Disney. That may level out once park reservations are removed next year for date based as at least people don't have to do that/know about that and get disappointed if they are blocked out of their park choice.
Over at Disney we do 5 days, we did 7 this past time but it was technically 6 day park ticket with hopper and 1 day with a CM. However, we were there for a wedding and large chunks of a few days were going to be unavailable so the longer days than usually just helped offset that.
I agree Disney tends to have more days but it's also 4 parks. With
Disneyland you don't usually see the norm being 5-7 days but rather 2-3 days with only having 2 parks so that is part of the puzzle.